photography, technology of


photography, technology of

Introduction

      equipment, techniques, and processes used in the production of photographs.

 The most widely used photographic process is the black-and-white negative–positive system (Figure 1—>). In the camera the lens projects an image of the scene being photographed onto a film coated with light-sensitive silver salts, such as silver bromide. A shutter built into the lens admits light reflected from the scene for a given time to produce an invisible but developable image in the sensitized layer, thus exposing the film.

      During development (in a darkroom) the silver salt crystals that have been struck by the light are converted into metallic silver, forming a visible deposit or density. The more light that reaches a given area of the film, the more silver salt is rendered developable and the denser the silver deposit that is formed there. An image of various brightness levels thus yields a picture in which these brightnesses are tonally reversed—a negative. Bright subject details record as dark or dense areas in the developed film; dark parts of the subject record as areas of low density; i.e., they have little silver. After development the film is treated with a fixing bath that dissolves away all undeveloped silver salt and so prevents subsequent darkening of such unexposed areas. Finally, a wash removes all soluble salts from the film emulsion, leaving a permanent negative silver image within the gelatin layer.

      A positive picture is obtained by repeating this process. The usual procedure is enlargement: the negative is projected onto a sensitive paper carrying a silver halide emulsion similar to that used for the film. Exposure by the enlarger light source again yields a latent image of the negative. After a development and processing sequence the paper then bears a positive silver image. In contact printing the negative film and the paper are placed face to face in intimate contact and exposed by diffused light shining through the negative. The dense (black) portions of the negative image result in little exposure of the paper and, so, yield light image areas; thin portions of the negative let through more light and yield dark areas in the print, thus re-creating the light values of the original scene.

Cameras (camera) and lenses

Basic camera functions
      In its simplest form, the camera is a light-tight container carrying a lens, a shutter, a diaphragm, a device for holding (and changing) the film in the correct image plane, and a viewfinder to allow the camera to be aimed at the desired scene.

      The lens projects an inverted image of the scene in front of the camera onto the film in the image plane. The image is sharp only if the film is located at a specific distance behind the lens. This distance depends on the focal length of the lens (see below Characteristics and parameters of lenses (photography, technology of)) and the distance of the object in front of the lens. To photograph near and far subjects, all but the simplest cameras have a focusing adjustment that alters the distance between the lens and the film plane to make objects at the selected distance produce a sharp image on the film. In some cameras focusing adjustment is achieved by moving only the front element or internal elements of the lens, in effect modifying the focal length.

      The shutter consists of a set of metallic leaves mounted in or behind the lens or a system of blinds positioned in front of the film. It can be made to open for a predetermined time to expose the film to the image formed by the lens. The time of this exposure is one of the two factors controlling the amount of light reaching the film. The other factor is the lens diaphragm, or aperture, an opening with an adjustable diameter. The combination of the diaphragm opening and exposure time is the photographic exposure. To obtain a film image that faithfully records all the tone gradation of the object, this exposure must be matched to the brightness (luminance) of the subject and to the sensitivity or speed of the film. Light meters built into most modern cameras measure the subject luminance and set the shutter or the lens diaphragm to yield a correctly exposed image.

Principal camera types
      The simplest camera type, much used by casual amateurs, has most of the features listed in the previous section—lens, shutter, viewfinder, and film-holding system. The light-tight container traditionally had a box shape. Present-day equivalents are pocket cameras taking easy-load film cartridges or film disks. Typically, a fixed shutter setting gives about 1/50-second exposure; the lens is permanently set to record sharply all objects more than about five feet (1.5 metres) from the camera. Provision for a flash may be built in. Though simple to handle, such cameras are in daylight restricted to pictures of stationary or slow-moving subjects.

The 35-mm miniature camera
      Perforated 35-millimetre (mm) film (originally standard motion-picture film) in cartridges holding 12 to 36 exposures with a nominal picture format of 24 × 36 mm is employed in miniature cameras. Smaller image formats down to 18 × 24 mm (half frame) may be used. The 35-mm camera has a lens with a range of apertures and a shutter with exposure times typically from one second to 1/1,000 second or shorter, and it can focus on subject distances from infinity down to five feet or less. A winding lever or built-in motor advances the film from one frame to the next and at the same time tensions (cocks) the shutter for each exposure. At the end of the film load the film is rewound into the cartridge for removal from the camera in daylight.

      A 35-mm camera usually has a direct-vision viewfinder, often combined with a rangefinder or autofocus system for accurate distance settings. Most current versions incorporate a light meter coupled with the exposure settings on the camera. Advanced models may have interchangeable lenses and an extended accessory system. Many 35-mm cameras are single-lens reflex types (see below).

The ultraminiature or subminiature
      This camera takes narrow roll film (16-mm or 9.5-mm) in special cartridges or film disks. The picture size ranges from 8 × 10 mm to 13 × 17 mm. These formats are used for making millions of snapshooting pocket-size cameras; special versions may be as small as a matchbox for unobtrusive use.

The view, or technical, camera
      For studio and commercial photography the view, or technical, camera takes single exposures on sheet films (formerly plates) usually between 4 × 5 inches and 8 × 10 inches. A front standard carries interchangeable lenses and shutters; a rear standard takes a ground-glass screen (for viewing and focusing) and sheet-film holders. The standards move independently on a rail or set of rails and are connected by bellows. Both standards can also be displaced laterally and vertically relative to each other's centre and swung or tilted about horizontal and vertical axes. These features provide versatility in image control (sharpness distribution, subject distance, and perspective), though not speed in use. The view camera is nearly always mounted on a tripod.

The medium-size hand camera
      This type of camera takes sheet film (typical formats of from 21/2 × 31/2 inches to 4 × 5 inches), roll film, or 70-mm film in interchangeable magazines; it has interchangeable lenses and may have a coupled rangefinder. Special types use wide-angle lenses and wide picture formats (e.g., 21/4 × 41/2 to 21/4 × 63/4 inches [6 × 12 to 6 × 17 centimetres]). The medium-size hand camera was popular with press photographers in the first half of the 20th century. Older versions had folding bellows and a lens standard on an extendable baseboard or strut system. Modern modular designs have a rigid body with interchangeable front and rear units.

The folding roll-film camera
      The folding roll-film camera, now rare, resembles the 35-mm miniature camera in shutter and viewfinder equipment but has bellows and folds up to pocketable size when not in use. Generally it takes roll films holding eight to 16 exposures; typical picture sizes are 21/4 × 21/4, 21/4 × 31/4, or 13/4 × 21/4 inches. Some 35-mm cameras were also produced with bellows.

The single-lens reflex
 The ground-glass screen at the back of the studio, or view, camera slows down picture taking because the screen must be replaced by the film for an exposure. The single-lens reflex camera (Figure 2—>) has a screen, but the film remains constantly in position. A 45° mirror reflects the image-forming rays from the lens onto a screen in the camera top. The mirror moves out of the way during the exposure and back again afterward for viewing and focusing the next picture. The image on the screen therefore temporarily disappears from view during the exposure. Present-day single-lens reflexes are either 35-mm cameras or advanced roll-film models. Most 35-mm reflexes have optical prism systems for eye-level screen viewing, built-in light-meter and electronic exposure-control systems, interchangeable lenses, and numerous other refinements. Often the camera is part of an extensive accessory system. Advanced roll-film reflexes are even more modular, with interchangeable viewfinders, focusing screens, and lenses.

The twin-lens reflex
 The twin-lens reflex is a comparatively bulky dual camera (Figure 3—>) with a fixed-mirror reflex housing and top screen mounted above a roll-film box camera. Its two lenses focus in unison so that the top screen shows the image sharpness and framing as recorded on the film in the lower section. The viewing image remains visible all the time, but the viewpoint difference (parallax) of the two lenses means that the framing on the top screen is not exactly identical with that on the film.

shutter and diaphragm systems
      Principal present-day shutters are the leaf shutter and the focal-plane shutter.

The leaf shutter
      The leaf, or diaphragm, shutter consists of a series of blades or leaves fitted inside or just behind the lens. The shutter opens by swinging the leaves simultaneously outward to uncover the lens opening. The leaves stay open for a fixed time—the exposure time—and then close again. A combination of electromagnets or electromagnets and springs drives the mechanism, while an electronic circuit—often coupled with a light metering system—or an adjustable escapement in mechanical shutters controls the open time. This is typically between one second and 1/500 second.

      The focal-plane shutter consists of two light-tight fabric blinds or a combination of metal blinds moving in succession across the film immediately in front of the image plane. The first blind uncovers the film and the second blind covers it up again, the two blinds forming a traveling slit the width of which determines the exposure time: the narrower the slit, the shorter the time. The actual travel time is fairly constant for all exposure times. A mechanism or electromagnet and control circuit triggers the release of the second blind. Focal-plane shutters are usually adjustable for exposure times between one second (or longer) and 1/1,000 to 1/4,000 second.

Diaphragm and shutter settings
      In the lens diaphragm a series of leaves increases or decreases the opening to control the light passing through the lens to the film. The diaphragm control ring carries a scale of so-called f-numbers, or stop numbers (relative aperture), in a series: such as 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, and 32. The squares of the f-numbers are inversely proportional to the amount of light admitted. In the above international standard series, each setting admits twice as much light as the next higher f-number, or stop (giving twice as much exposure).

      Shutter settings on present-day cameras also follow a standard double-or-half sequence—e.g., 1, 1/2, 1/4, 1/8, 1/15, 1/30, 1/60, 1/125, 1/250, 1/500, 1/1,000 second, and so forth. The shorter the exposure time, the “faster” the shutter speed.

Exposure values
      An attempt to simplify the mathematics of f-number and shutter speed-control functions led to the formulation of exposure values (EV). These run in a simple whole-number series, each step (EV interval) doubling or halving the effective exposure. The lower the EV number, the greater the exposure. Thus, EV 10 gives twice as much exposure as EV 11 or half as much as EV 9. Each EV value covers a range of aperture/speed combinations of the same equivalent exposure; for instance, f/2.8 with 1/250 second, f/4 with 1/125 second, and f/5.6 with 1/60 second. For a time some cameras carried an EV scale and coupled the aperture and speed settings; at a given EV setting in such cameras selecting various speeds automatically adjusted the aperture to compensate and vice versa. Exposure-value setting scales became obsolete with exposure automation, but the notation remains in use to indicate either exposure levels or—at specified film speeds—lighting levels requiring a given exposure.

Automatic-diaphragm systems
      On a camera with a viewing screen (view camera or single-lens reflex) viewing and focusing are carried out with the lens diaphragm fully open, but the exposure is often made at a smaller aperture. Reflex cameras (and increasingly also view cameras) therefore incorporate a mechanism that automatically or semiautomatically stops down (reduces) the lens to the working aperture immediately before the exposure.

Methods of focusing and framing
      The ground-glass (now mostly grained plastic) screen is the most direct way of viewing the image for framing and for sharpness control. The screen localizes the image plane for observation. The image is also visible without a screen, but then the eye can locate the image plane of maximum sharpness only with a precisely focused high-power magnifier. This aerial focusing method avoids interference of the ground-glass structure with sharpness assessment.

Focusing aids
      The eye is not good at recognizing slight unsharpness, so focusing screens (especially in reflex cameras) often incorporate focusing aids such as a split-image wedge alone or with a microprism area, in the screen centre. The split-image wedge consists of a pair of prism wedges that split an out-of-focus image into two sharp halves laterally displaced relative to one another. When the lens is correctly focused the image becomes continuous across the wedge area—a point that the eye can assess more precisely. The microprism area contains several hundred or thousand minute wedges that give a blurred image very ragged outlines and a broken-up texture; these clear abruptly as the image becomes sharp.

      The focusing screen is often overlaid by a pattern of fine concentric lens sections. Called a Fresnel screen, it redirects the light from the screen corners toward the observer's eye and makes the image evenly bright.

      Cameras without a screen generally are equipped with a distance scale, the lens being set to the estimated object distance. More advanced cameras have an optical rangefinder (range finder) as a distance-measuring aid; it consists of a viewfinder (see below) and a swinging mirror a few inches to one side of the viewfinder axis. As the eye views an image of the object, the mirror superimposes a second image from a second viewpoint. On turning the mirror through the correct angle, which depends on the object distance, the two images are made to coincide. The mirror movement can be linked with a distance scale, or coupled with the lens focusing adjustment. When the lens is incorrectly focused, the rangefinder shows a double or split image. In place of a rotating mirror, the rangefinder may use swinging or rotating optical wedges (prisms).

      Some cameras evaluate the coincidence (or lack thereof) between two rangefinder images by image analysis with a microchip system. This signals electronically when the lens is set to the correct distance and often carries out the distance setting by a servomotor built into the camera. Such focusing automation makes the camera even simpler to use. Alternative automatic ranging systems used in amateur cameras depend on triangulation with infrared rays or pulses sent out by a small light-emitting diode (LED), or on measurement of the time an ultrasonic signal takes to be reflected back from the subject ( sonar).

      While these devices measure distance automatically, single-lens reflex cameras may incorporate electronic image-analysis systems to measure sharpness. The signal output of such systems actuates red or green LEDs in the camera finder system to show whether the image is sharp or not. The same signal can control a servomotor in the lens for fully automatic focusing. These devices are limited at low lighting and contrast levels—where the human eye also finds sharpness assessment difficult.

Viewfinders (viewfinder)
      The sighting devices in cameras lacking screens are called viewfinders; they show how much of the scene will appear on the film. The simplest viewfinder is a wire frame above the camera front, with a second frame near the back to aid the eye in correct centring. Most present-day finders are built into the camera and are compact lens systems. Bright-frame finders show a white frame reflected into the view to outline the field recorded on the film. An alternative form is the reflecting viewfinder in which the photographer looks down into a field lens on top of the camera. The upper section of a twin-lens reflex camera is such a reflecting finder.

      As the viewfinder axis in a camera other than a single-lens reflex does not usually coincide with the lens axis, the finder's and the lens's views do not exactly match. This parallax error is insignificant with distant subjects; with near ones it is responsible for the familiar fault of a portrait shot of a head that appears partly cut off in the picture even though it was fully visible in the finder. Camera viewfinders may have parallax-compensating devices.

      The optical finder gives a direct upright and right-reading view of the subject with the camera held at eye level. The traditional reflex camera, held at waist level, showed a laterally reversed view. Modern reflexes have a pentaprism arrangement that permits upright, right-reading, eye-level viewing by redirecting the image from the horizontal screen on top of the camera.

Exposure-metering systems
      Exposure meters (exposure meter), or light meters, measure the light in a scene to establish optimum camera settings for correct exposures. A light-sensitive cell (photoelectric cell) generates or controls an electric current according to the amount of light reaching the cell. The current may energize a microammeter or circuit controlling LEDs to indicate exposure settings. In most modern cameras the current or signal acts on a microprocessor or other circuit that directly sets the shutter speed or lens aperture. The cell usually is a silicon or other photodiode generating a current that is then amplified. In older cadmium sulfide cells the light falling on the cell changed the latter's resistance to a current passing through it. selenium cells, still used in some cameras, also generate a current but are larger and less sensitive.

      Single-lens reflex cameras have one or more photocells fitted in the pentaprism housing to measure the brightness of the screen image. The exposure reading depends on the light coming through the lens (TTL metering) and so allows for the lens's angle of view, close-up exposure corrections, stray light, and other factors. Some TTL systems divert the light from the lens to a photocell before it reaches the screen (e.g., by beam-splitting arrangements or the use of photocells behind a partly reflecting mirror), or they measure the light reflected from the film or from a specially structured first shutter blind at the beginning of, or during, the exposure. Such off-the-film (OTF) measurement is also used for electronic flash control (see below).

      View cameras may use a photocell on a probe that can be moved to any point just in front of the focusing screen, thus measuring image brightness at selected points of the image plane. This takes place before the exposure, and the probe is then moved out of the way. Professional photographers also use hand-held separate exposure meters and transfer the readings manually to the camera.

Flash (flash lamp) systems
      Flash is a widely used artificial light source for photography, providing a reproducible light of high intensity and short duration. It can be synchronized with an instantaneous exposure. Being battery powered, small flash units are self-contained.

      The most common flash system depends on a high-voltage discharge through a gas-filled tube. A capacitor charged to several hundred volts (by a step-up circuit from low-voltage batteries or from the line voltage supply) provides the discharge energy. A low-voltage circuit generating a high-voltage pulse triggers the flash, which lasts typically 1/1,000 second or less. Small electronic flash units may be built into or clipped onto the camera. Larger units are attached with brackets. Large professional units with floodlight and spotlight fittings are used in studio photography. Even small flashes often have adjustable reflectors, for example, to illuminate an indoor subject by the flash reflected from the ceiling or walls.

Automatic and dedicated flash
      Electronic flash units often incorporate a fast-responding photodiode that cumulatively measures the light reflected from the subject and switches off the flash when that light has reached a preselected amount (computer flash). This flash-duration control thus adjusts the flash exposure automatically as long as the subject is within a certain distance range (typically from two to 20 feet). At lower power or near subject distances the duration of a computer flash may drop to 1/50,000 second.

      With certain camera–flash combinations OTF metering inside the camera can control the flash duration by suitable contacts made when the flash is attached to the camera. These “dedicated” flashes (so named because their control circuitry has to match that of specific cameras) may also signal in the camera finder when the flash is ready to operate and to set the camera automatically to its synchronizing shutter speed (see below).

Flashbulbs (flashbulb)
      An older type of flash is an oxygen-filled glass envelope containing a specific amount of aluminum or zirconium wire and means for igniting the wire in the bulb. The wire burns away with a brilliant flash lasting typically about 1/100 to 1/50 second. Each flashbulb can, however, yield only one flash. Current flashbulb systems use four to 10 tiny bulbs, each in its own reflector, arranged in cube or bar carriers that plug into cameras designed for them. The individual flashes are fired in turn by a battery and circuit in the camera through mechanically generated current pulses or other means. In view of the greater convenience of electronic flash, flashbulbs in their various forms are largely obsolescent.

Firing and synchronization
      Flash units are usually fired with a switch in the camera shutter to synchronize the flash with the shutter opening. A contact in the camera's flash shoe (hot shoe) or a flash lead connects the unit with this shutter switch. The shutter contact usually closes the instant the shutter is opened. A focal plane shutter must fully uncover the film (generally at a shutter speed of 1/60 second or slower) for flash synchronization. With flashbulbs the shutter must also stay open while the flash reaches its peak brightness—about 1/50 second.

System cameras
      From the development of the 35-mm miniature camera in the 1930s evolved the concept of the system camera that could be adapted to numerous jobs with a range of interchangeable components and specialized accessories. Today, most moderately advanced 35-mm miniatures take interchangeable lenses, close-up and photomicrographic attachments, filters, flash units, and other accessories. The most elaborate camera systems also include such accessories as alternative finder systems; interchangeable reflex screens, film backs, and magazines; and remote-control and motor-drive systems. Modular professional roll-film and view cameras are built up from a selection of alternative camera bodies, film backs, bellows units, lenses, and shutters. This is the nearest approach to the universal camera, assembled as required to deal with practically every type of photography.

Characteristics and parameters of lenses (lens)
      The lens forming an image in the camera is a converging lens, the simplest form of which is a single biconvex (lentil-shaped) element. In theory such a lens makes a light beam of parallel rays converge to a point (the focus) behind the lens. The distance of this focus from the lens itself is the focal length, which depends on the curvature of the lens surfaces and the optical (optics) properties of the lens glass. An object at a very long distance (optically regarded as at “infinity”) in front of the lens forms an inverted image in a plane (the focal plane) going through the focus. Light rays from nearer objects form an image in a plane behind the focal plane. The nearer the object, the farther behind the lens the corresponding image plane is located—which is why a lens has to be focused to get sharp images of objects at different distances.

Focal length and image scale
 The image scale, or scale of reproduction, is the ratio of the image size to the object size; it is often quoted as a magnification. When the image is smaller than the object, the magnification of the object is less than 1.0. If the image is 1/20 the size of the object, for example, the magnification may be expressed either as 0.05 or as 1:20. For an object at a given distance, the scale of the image depends on the focal length of the lens (Figure 4—>). A normal camera lens usually has a focal length approximately equal to the diagonal of the picture format covered. A lens of longer focal length gives a larger scale image but necessarily covers less of the scene in front of the camera. Conversely, a lens of shorter focal length yields an image on a smaller scale but—provided the angle of coverage is sufficient (see below)—takes in more of the scene. Many cameras, therefore, can be fitted with interchangeable lenses of different focal lengths to allow varying the image scale and field covered. The focal length of a lens in millimetres (sometimes in inches) is generally engraved on the lens mount.

      The aperture, or f-number, is the ratio of the focal length to the diameter of an incident light beam as it reaches the lens. For instance, if the focal length is 50 millimetres and the diameter of the incident light beam is 25 millimetres, the f-number is 2. This incident-beam diameter is often roughly the lens-diaphragm diameter, but it may be appreciably larger or smaller. The maximum aperture (f-number at the largest diaphragm opening) is also marked on the lens, usually in the form f:2, f/2, or 1:2.

Angle of coverage
      A lens must cover the area of a camera's film format to yield an image adequately sharp and with reasonably even brightness from the centre to the corners of the film. A normal lens should cover an angle of at least 60°. A wide-angle lens covers a greater angle—about 70° to 90° or more for an ultrawide-angle lens. A long-focus lens covers a smaller angle.

      The angle of coverage depends on the lens design. Designations like “wide angle” or “narrow angle” are not necessarily synonymous with “short focus” and “long focus,” as the latter terms refer to the focal length of the lens relative to the picture format.

Optical (optical image) performance
      A simple lens produces a very imperfect image, which is usually blurred away from the centre. The image may have colour fringes around object outlines, and straight lines may be distorted. Such defects, called aberrations (aberration), can be eliminated—and even then not completely—only by replacing the single lens element by a group of elements of appropriate shape and separation. Aberrations arising from some of the lens elements then counteract opposite aberrations produced by other elements. The larger the maximum aperture, the greater the angle of coverage, and the higher the degree of correction aimed at, the more complex camera lenses become. Lens design for relative freedom from aberrations involves advanced computer programming to calculate the geometric parameters of every lens element. Some aberrations can also be corrected by making one or more of the surfaces of a lens system aspheric; i.e., with the variable curvature of a paraboloid or other surface rather than the constant curvature of a spherical one.

      Lenses usually consist of optical glass. Transparent plastics also have come into use, especially as they can be molded into elements with aspheric surfaces. They are, however, more sensitive to mechanical damage.

Aberrations
      There are a number of lens aberrations, each with its own characteristics. chromatic aberration is present when the lens forms imagesby different-coloured light in different planes and at different scales. Colour-corrected lenses largely eliminate these faults. Spherical aberration is present when the outer parts of a lens do not bring light rays into the same focus as the central part. Images formed by the lens at large apertures are therefore unsharp but get sharper at smaller apertures. Curvature of field is present when the sharpest image is formed not on a flat plane but on a curved surface. Astigmatism occurs when the lens fails to focus image lines running in different directions in the same plane; in a picture of a rail fence, for instance, the vertical posts are sharp at a focus setting different from the horizontal rails. Another aberration, called coma, makes image points near the edges of the film appear as irregular, unsharp shapes. Distortion is present when straight lines running parallel with the picture edges appear to bow outward (barrel distortion) or inward (pincushion distortion).

Resolving power and contrast-transfer function
      One way of testing lens performance is to observe the image it forms of patterns of increasingly closely spaced black lines separated by white spaces of line width. The closest spacing still recognizable in the image gives a resolving power value, expressed in line pairs (i.e., black line plus white space) per millimetre. Photographs of such line patterns, or test targets, show the resolving power of the lens and film combination. For example, a resolution of 80–100 line pairs per millimetre on a fine-grain film represents very good performance for a normal miniature camera lens.

      The visual sharpness of an image depends also on its contrast. Opticians, therefore, often plot the contrast with which the image is reproduced against the line spacing of that image. The resulting contrast-transfer curve, or function, gives a more reliable indication of the lens performance under practical picture-taking conditions.

Special lens types
      Apart from general-purpose camera lenses of various focal lengths, there are lenses of special characteristics or design.

Telephoto lenses
      Long-focus lenses are bulky, because they comprise not only the lens itself but also a mount or tube to hold it at the appropriate focal distance from the film. Telephoto lenses are more compact; their combinations of lens groups make the back focus (the distance from the rear lens element to the film) as well as the length of the whole lens appreciably shorter than the focal length. Strictly, the term telephoto applies only to a lens of this optically reduced length; in practice long-focus lenses of all types tend to be called indiscriminately telephoto or “tele” lenses.

      If a camera lens is interchangeable, an accessory teleconverter lens group can be positioned between the prime lens and the camera. This turns a normal lens into an even more compact telephoto system, which is less costly than a telephoto lens but which reduces the speed of the prime lens and usually impairs sharpness performance.

Wide-angle and retrofocus lenses
      Short-focus, wide-angle lenses are usually mounted near the film. Single-lens reflex cameras need a certain minimum lens-to-film distance to accommodate the swinging mirror. Wide-angle (and sometimes normal-focus) lenses for such cameras therefore use retrofocus designs. In these the back focus is appreciably longer than the focal length. Both a telephoto and a retrofocus lens must be specially designed for its particular use to ensure optimum image performance.

Fish-eye lenses
      For image angles greater than 110°, it becomes difficult to bring the lens close enough to the film to allow the rays between the lens and film to diverge sufficiently. The fish-eye lens overcomes this difficulty by making the rays diverge less behind the lens than they do in front. The resulting image shows appreciable distortion, with image details near the edges and corners progressively compressed. Fish-eye lenses usually cover angles between 140° and 210° and are used for unusual wide-angle effects where the distortion becomes a deliberate pictorial element. They also have certain scientific applications, for instance, to cover a horizon-to-horizon view of the sky in recording cloud formations.

Mirror lenses
      Images can also be formed by light reflected from curved mirrors. This method, long used in astronomical telescopes, is applied to long-focus lens systems of short overall length by folding the light path back onto itself. A mirror lens or catadioptric system has no chromatic aberrations. Other aberrations are corrected by incorporating one or more appropriate lens elements. The arrangement of the system, with a central opening in the primary mirror, makes stopping down with a customary diaphragm difficult, and neutral-density filters are used to control light transmission.

Variable-focus lenses
      In variable-focus lenses the focal length can be varied by movement of some of the elements or groups within the lens system. One lens can thus replace a range of interchangeable lenses.

      The variable-focus, or zoom, lens was originally developed for motion-picture photography, in which adjustment of the focal length during a shot produced a zooming-in or zooming-out effect (hence the name). It is now widely used in single-lens reflex cameras where the reflex finder permits accurate continuous assessment of image coverage. In a true zoom lens the image changes in scale but not in sharpness during zooming; some varifocal lenses, however, need refocusing at different focal lengths. Due to correction requirements over a range of focal lengths, zoom lenses are complex systems containing from 12 to 20 elements. Zoom lenses for still cameras have focal-length ratios from 2:1 to 4:1 or more (e.g., 35–135 mm for a 35-mm reflex).

Lens-changing systems
      Miniature and roll-film cameras hold interchangeable lenses in screw or quick-change bayonet mounts. In a focal-plane shutter camera the usable range of focal lengths is practically unlimited. In cameras with leaf shutters, either the lens is mounted in front of the shutter or the lens is changed with the shutter. Some designs use convertible lenses with the rear components built into the camera together with the shutter; interchangeable front groups then provide different focal lengths in combination with the fixed rear group. View-camera lenses—usually with their own shutters—are mounted on lens boards that clip into and out of the front camera standard.

      Afocal attachments provide the effect of alternative focal lengths with a fixed camera lens. They are magnifying or reducing telescopes without a focal length (hence afocal), yielding a virtual image that the camera lens projects onto the film. Their designated magnification factor indicates the effect on the image scale; e.g., a 1.5× tele attachment magnifies the image on the film 11/2 times, while a 0.7× wide-angle attachment reduces the image scale to 0.7 times that of the prime camera lens.

Lens coating
      When light passes from one optical medium to another (especially from air to glass and vice versa in a lens), about 4 to 8 percent of it is lost by reflection at the interface. This light loss builds up appreciably in complex multielement lenses. Some of the reflected light still reaches the film as ghost images or light spots or as general contrast-reducing scattered light.

      To reduce such losses, the air-to-glass surfaces of modern lenses typically carry a microscopically thin coating of metallic fluorides. The coating eliminates most reflected rays. Complete elimination can occur only for light of one wavelength if the coating thickness and refractive index are exactly right. In practice a coated lens surface reflects about 0.5 percent of incident white light—1/10 of the light lost by an uncoated lens. Multiple coatings can reduce reflections over a wider wavelength range.

Black-and-white films

The latent image
      The sensitive surface of ordinary film is a layer of gelatin carrying minute suspended silver halide crystals or grains (the emulsion)—typically silver bromide with some silver iodide. Exposure to light in a camera produces an invisible change yielding a latent image, distinguishable from unexposed silver halide only by its ability to be reduced to metallic silver by certain developing agents.

      Current theories postulate that silver halide crystals carry minute specks of metallic silver—so-called sensitivity specks—which amount in mass to about 1/100,000,000 part of the silver halide crystal. A silver halide is a compound of silver with fluorine, chlorine, bromine, or iodine, but only the last three are light-sensitive. When light action releases electrons from the silver halide crystal, they migrate to the sensitivity specks. The resulting electric charge on the specks attracts silver ions from the neighbouring silver halide; and as the silver ions accumulate, they become metallic silver, causing the speck to grow. Halogen (e.g., bromine) atoms at the same time migrate to the surface of the silver halide crystal and are there absorbed by the gelatin of the emulsion. When the sensitivity speck is large enough, it provides a point of attack for the developer, which can then reduce the whole silver halide crystal to silver. Developers are selective organic reducing agents that attack only silver halide crystals that have sufficiently large sensitivity specks. The halide grains carrying a developable sensitivity speck make up the latent image.

Sensitometry and speed
      The sensitivity or speed of a film determines how much light it needs to produce a given amount of silver on development. Sensitometry is the science of measuring this sensitivity, which is determined by giving the material a series of graduated exposures in an appropriate instrument (the sensitometer). After development under specified conditions, the density of the silver deposit produced by each exposure is measured and the densities are plotted on a graph against the logarithm of the exposure. The resulting characteristic curve, or D/log E curve (see below Contrast (photography, technology of)), shows how the film reacts to exposure changes. A specified point on the curve also serves as a criterion for calculating film speed by methods laid down in various national and international standards.

      The internationally adopted scale is ISO speed, written, for example, 200/24°. The first half of this (200) is arithmetic with the value directly proportional to the sensitivity (and also identical with the still widely used ASA speed). The second half (24°) is logarithmic, increasing by 3° for every doubling of the speed (and matching the DIN speeds still used in parts of Europe). A film of 200/24° ISO is twice as fast (and for a given subject requires half as much exposure) as a film of 100/21° ISO, or half as fast as a film of 400/27° ISO.

      All-around films for outdoor and some indoor photography have speeds between 80/20° and 200/24° ISO; fine-grain films for maximum image definition between 25/15° and 64/19° ISO; and high-speed and ultraspeed films for poor light from 400/27° ISO up.

Colour sensitivity
      Initially, the silver halide emulsion is sensitive to ultraviolet radiation and to violet and blue light. Most films contain sensitizing dyes to extend their colour sensitivity through the whole visible spectrum. Such films, called panchromatic films, were introduced in 1904. They record subject colour values as gray tones largely corresponding to the visual brightness of the colours.

      Non-colour-sensitized or blue-sensitive emulsions (without sensitizing dyes) are used for copying monochrome originals and similar applications needing no extended colour sensitivity. At one time orthochromatic films—sensitive to violet, blue, green, and yellow but not to red—were also used for general photography; now they are employed mainly for photographing of phosphor screens, such as cathode-ray tubes, and for other purposes requiring green but not red sensitivity.

      Infrared films, developed in 1919, are sensitized to invisible infrared wavelengths. They are used in aerial photography to cut through atmospheric haze (which scatters blue light but not infrared rays) and for special purposes in scientific and forensic photography.

Filters (filter)
      Filters can modify the way in which a film records colours as monochrome tone values. They are disks of coloured glass or gelatin with controlled transmission characteristics. Placed in front of the camera lens, they preferentially transmit light of their own colour and hold back light of other colours. A yellow or yellow-green filter is often used in landscape photography to prevent overexposure of the blue sky and to bring out detail in cloud formations. Orange and red filters make the sky still darker and cut through haze by absorbing scattered blue light.

      Contrast filters differentiate between the gray values of objects of different colour but of similar brightness. For instance, a red flower and green foliage record in similar shades of mid-gray. A red filter holds back green light to darken the green foliage, making the flower lighter; a green filter absorbs red light, thus darkening the flower. Such deliberate tone distortion is widely used in photomicrography and other fields.

      Other filter types used in photography include ultraviolet, infrared, and polarizing filters. Ultraviolet-absorbing filters screen out ultraviolet rays at high altitudes (e.g., in mountain photography). Because camera lenses are not normally corrected for such rays, the rays can reduce image sharpness, even though the lenses allow only a small amount of ultraviolet to be transmitted. Infrared filters are used with infrared film to hold back visible light. Polarizing filters polarize light and can absorb polarized light if suitably oriented. Light reflected at certain angles from shiny surfaces of nonmetallic media (glass, water, varnish) is polarized; a properly oriented polarizing filter subdues such reflections in a picture.

      Because a filter screens out part of the light, its use calls for extra exposure, the amount of which is indicated by a filter factor—e.g., 2×, which means the exposure time must be multiplied by 2. For cameras with an exposure-value scale, a filter may specify an exposure value reduction (such as -1 or -11/2; i.e., the indicated exposure value must be reduced by this amount). The factor of a given filter depends on the spectral sensitivity of the film, the colour quality of the lighting, the type of subject, the effect aimed at, and other exposure conditions.

Other film characteristics
      Of practical interest to the photographer are the graininess, resolving power, and contrast of a film. Although they are characteristics of the film itself, they are influenced by the conditions of development (see below Black-and-white processing and printing (photography, technology of)).

      The image derived from minute silver halide crystals is discontinuous in structure. This gives an appearance of graininess in big enlargements. The effect is most prominent with fast films, which have comparatively large silver halide crystals.

Resolving power and acutance
      The fineness of detail that a film can resolve depends not only on its graininess but also on the light scatter or irradiation within the emulsion (which tends to spread image details) and on the contrast with which the film reproduces fine detail. These effects can be measured physically to give an acutance value, which is preferred to resolving power as a criterion of a film's sharpness performance. Fine-grain films with thin emulsions yield the highest acutance.

 High-contrast films reproduce tone differences in the subject as great density differences in the image; low-contrast films translate tone differences into small density differences. The characteristic curve of a film obtained by plotting the density against the logarithm of the exposure (mentioned earlier under Sensitometry and speed (photography, technology of)) can be used to express a film's contrast (see Figure 5—>). The slope of the straight-line section of the curve (sometimes called the gamma, actually the tangent of the angle α) indicates contrast: the steeper the slope, the higher the contrast rendering. General-purpose films yield medium contrast (gamma 0.7 to 1). High-contrast films (gamma 1.5 to 10) are used for copying line originals and other specialized purposes; low-contrast films for continuous-tone reproduction. Gamma is also used to indicate degree of development, since increased development generally results in a higher gamma.

Film structure and forms
      Film consists of a number of layers and components: (1) A supercoat of gelatin, a few micrometres (one micrometre is 0.001 millimetre) thick, protects the emulsion from scratches and abrasion marks. (Pressure and rubbing can produce developable silver densities.) (2) The emulsion layer (silver halide suspended in gelatin) is usually nine to 12 micrometres (up to 1/2,000 inch) thick but may sometimes reach 25 micrometres. (3) A substrate or subbing layer promotes adhesion of the emulsion to the film base. (4) The film base, or support, is usually cellulose triacetate or a related polymer. The thickness may range from 0.08 to 0.2 millimetre (0.003 to 0.008 inch). Films for graphic arts and scientific purposes are often coated on a polyethylene terephthalate or other polyester support of high dimensional stability. Glass plates—once the most common support for negative materials—are now used only for applications requiring extreme emulsion flatness. (5) A backing layer on the rear of the film base counteracts curling. Usually it contains also a nearly opaque dye to suppress light reflection on the rear support surface. Such reflection (halation) reduces definition by causing halolike effects around very bright image points. Some film bases (especially in 35-mm films) are tinted gray to absorb light that has passed through the emulsion layer.

      View and studio cameras generally take sheet film—single sheets (typical sizes range between 21/2 × 31/2 and 8 × 10 inches) loaded in the darkroom into light-tight film holders for subsequent insertion in the camera.

      The term roll film is usually reserved for film wound up on a spool with an interleaving light-tight backing paper to protect the wound-up film. The spool is loaded into the camera in daylight, the backing paper leader threaded to a second spool, and the film wound from picture to picture once the camera is closed. This is the classical roll film of roll-film cameras. Common current film widths are 62 mm and 45 mm. The rear of the backing paper carries sets of consecutive numbers spaced at frame intervals for different image formats. In some roll-film cameras these numbers are visible through a viewing window in the camera and show how far the film must be wound to advance it from one picture to the next. Instant-loading cartridges also use paper-backed roll film.

Perforated film
      Some film is perforated along its edges and rolled up on its own inside a light-tight cartridge, which can be loaded into the camera in daylight. Once the camera is closed, a transport sprocket engaging the edge perforations draws the film from the cartridge onto a spool and advances it from picture to picture. The most common film width is 35 mm (for 35-mm miniature cameras), and its cartridge typically holds enough film for up to 36 (sometimes 72) exposures. A 70-mm film for larger cameras and 16-mm strips for ultraminiatures are packed and used in a similar way.

      In March 1983 the Eastman Kodak Company announced the development of a new coding system for 35-mm film and cartridges. The DX film system employs optical, electrical, and mechanical encoding to transmit to appropriately equipped cameras such information as film type, film speed, and number of exposures. The system also supplies data that enable automatic photofinishing equipment to identify and sort film quickly, simplifying processing and printing. In the interest of uniformity, Kodak freely offered the DX system to all film and camera manufacturers, and within two years it was generally adopted.

Disk film
      Some compact mass-market cameras take circular disks of film, 65 millimetres in diameter, in light-tight cartridges and coated on a 0.18-mm polyester base. In the camera the disk rotates as up to 15 exposures (frame size 8 × 10 millimetres) are recorded around the disk circumference. The disk lies flatter in the camera than rolled-up film and is suitable for more automated photofinishing; the high printing magnification required, however, limits the image quality.

Picture-taking technique
      The main areas of practical camera handling in photography concern sharpness control, exposure, and lighting.

Sharpness control
      The image on the film is sharpest when the lens is focused to the exact object distance. Usually, however, a scene includes objects at varying distances from the camera. Various factors affect the sharpness distribution in a picture of such a scene.

      The sharpness in the image of objects in front of and behind the focused distance falls off gradually. Within a certain range of object distances this sharpness loss is still comparatively unnoticeable. This range is the depth of field and depends on: (1) the amount of sharpness loss regarded as acceptable: miniature negatives requiring big enlargement must be sharper than larger format negatives, which are enlarged less; (2) the lens aperture used: stopping down the lens (higher f-numbers) increases the depth of field; (3) the object distance: the depth of field is smaller for near objects than for more distant ones; and (4) the focal length of the lens: depth of field is reduced with longer focus lenses (and with larger picture formats requiring lenses of longer focal length), and the depth increases with shorter focus lenses. A depth of field indicator, often included on the focusing mounts of lenses, shows on the distance scale how far in front of and behind the focused distance objects will be in focus at different diaphragm openings.

Subject and camera movement
      Movement of the subject while the camera shutter is open for the exposure leads to a blurred image. The exposure time must therefore be short enough to keep the blur within acceptable limits. The shutter speed required depends on the movement speed of the object, the scale of the image (movement blur becomes greater the nearer the subject or the longer the focal length of the lens used) and the movement direction; movement across the direction of view produces the most blurring.

      Movement blur can be reduced, even with comparatively slow shutter speeds, by moving the camera (panning) to follow the subject during the exposure. This records the moving object comparatively sharply against a blurred background and emphasizes the impression of speed.

      Camera shake through unsteady support during the exposure also creates image blur—over the whole picture in such cases. Hand-held shots generally demand shutter speeds of 1/30 second or shorter. For longer times a firm camera support—such as a tripod—is essential.

Exposure technique
      The correct exposure (aperture and shutter settings) can be derived from tables or calculators or by direct measurement of the subject luminance with a light meter.

Automatic meter control
      Cameras with through-the-lens (TTL) exposure meters—and also hand-held meters pointed at the subject—measure the average reflected light intensity, yielding reliable exposures for subjects of average contrast and brightness distribution. Subjects of extreme contrast or very bright or dark dominant areas need overriding exposure corrections; automatic cameras often have provision for this. Such a TTL measurement is usually centre-weighted (predominantly based on the image centre). Some cameras (and meters) permit spot readings covering a small subject area only and give reliable exposures if this selected area is a medium subject tone.

      The selection of an appropriate aperture and shutter speed among equivalent camera exposures depends on depth-of-field and subject-movement requirements. Some automatic cameras simplify this by selecting just one such combination at each exposure level (program automation).

Flash (flash lamp) exposures
      Most current electronic flash units incorporate a sensor cell that measures the light reflected from the subject and controls the flash duration (and hence the exposure) accordingly. In certain cameras in which photocells measure the light reflected from the film, the same cells can similarly control the flash duration of suitable dedicated flash units. Lacking these provisions, flash exposures may be determined by measurement or by guide-number calculation.

      Special meters can measure flash light quantity on a scene during a test firing of flashes; these are used extensively with more elaborate studio setups.

      Flash exposure calculations rely on the fact that the exposure depends only on the lens aperture. (The electronic flash is usually much shorter than the synchronizable shutter time.) The light intensity reaching the film is inversely proportional to the square of the diaphragm f-number. By basic illumination laws the light intensity on a scene is also inversely proportional to the square of the distance between the light source and subject. For a given flash source and film speed, the exposure is thus constant for a constant product of distance and f-number. Flash manufacturers quote this product as a guide number for various flash–film combinations. For rapid exposure calculation, dividing the guide number by the flash-to-subject distance gives the required f-number; dividing the guide number by the f-number gives the distance at which the flash must be arranged for correct exposure.

      Some cameras use this principle for semiautomatic flash-exposure control: the aperture adjustment is coupled with the distance setting on the lens (or with an automatic rangefinding system) so that the lens aperture gets larger with increasing distance. This coupling is adjustable for different flash guide numbers.

Exposure latitude
      The ideal negative exposure records the darkest subject shadows as a just visible density. More exposure yields a denser negative, which, however, can still give an acceptable print by appropriate print-exposure adjustment. This range of usable negative exposures, the exposure latitude, depends on the film and the subject. This latitude is greater the lower the subject contrast and the greater the film's exposure range (and, generally, the lower the film contrast). Because of exposure latitude, simple cameras with limited exposure adjustability can still yield acceptable pictures under differing light conditions.

Lighting technique
      The kind of lighting on the scene governs the way in which the picture reproduces the subject. Orientation of the subject—as in taking a portrait—with respect to the light direction can often control the effect. Lighting from behind the camera gives flat effects, light from one side yields depth and modeling, while the principal light from behind the subject produces dramatic against-the-light effects of high contrast. Artificial light setups in the studio, with tungsten lamps or electronic flash, offer the greatest flexibility. Under such conditions the photographer can arrange two or more lamps for various lighting effects.

      Directional lighting improves detail contrast and brilliance. Excessive subject contrast, however, makes accurate exposure settings difficult and may lead to loss of picture detail in the highlights or shadows. Fill-in lighting, by a flash or other light source on or near the camera, can illuminate heavy shadows facing the camera.

Black-and-white processing and printing

Negative development
      Amateurs usually process films in developing tanks. In this type of development roll or miniature film is wound around a reel with a spiral groove, which keeps adjacent turns separated and allows access by the processing solutions. Once the tank is loaded (in the dark), processing takes place in normal light, the processing baths (developer, intermediate rinse, fixer) being poured into the tank at the appropriate intervals. Sheet films are similarly treated in small tanks or held in hangers and immersed sequentially in the different processing solutions. Large-scale commercial processing laboratories use machines that automatically feed the films through the solutions in proper sequence.

Developers and their characteristics
      The developer consists typically of one or more developing agents, a preservative (such as sodium sulfite) to prevent oxidation by the air, an alkali (such as sodium carbonate) to activate the developer, and a restrainer or antifoggant to ensure that the developer acts only on exposed silver halide crystals. A developer's main characteristics are activity, development speed, and effect on film gradation, graininess, and sharpness. Developers may be prepared on the basis of published formulas or bought as ready-mixed powders or concentrates for dilution with water.

      The developer is allowed to act for a specific time to build up the image to the required density and contrast. This time depends on the developer, the temperature, the degree of agitation, and the film—as indicated by recommendations from film and developer manufacturers.

      The fixing bath contains a chemical (sodium or ammonium thiosulfate) that converts the silver halide into soluble, complex silver salts that dissolve in the fixer. During this process the film loses its original silver halide milkiness overlaying the image and becomes clear. The fixer also contains a weak acid (to halt the development process) and a hardening agent to reduce gelatin swelling.

Washing and drying
      Washing removes all residual soluble chemicals from the emulsion and must be thorough for image permanence. Films are hung up to dry after removal from the tank.

High-speed processing
      Greatly reduced processing times are possible with high-activity developers at elevated temperatures and with fast-acting fixing agents, such as ammonium thiosulfate. Such processes can cut access time to the negative down to less than a minute. One-bath (monobath) processing in a solution containing both a fast-acting developing agent and fixing chemicals also reduces processing time. In special rapid-access processing equipment, films pass through chambers spraying the processing solutions onto the film surface or run in contact with monobath-soaked webs.

Printing
      The simplest printing equipment is the contact printing frame in which the negative and printing paper are held together behind a glass plate during exposure to a suitable lamp. A printing box is essentially a printing frame with a built-in light source. Contact printing gives a positive of the same size as the negative.

Enlargers (enlarger)
      Negatives usually are enlarged to prints of the desired final size. The enlarger is a projection system on a vertical column mounted on a horizontal baseboard. It has a lens, a film holder (negative carrier), and a lighting system (typically a lamp and condenser lens) for illuminating the negative. Raising or lowering the enlarger head on the column controls the image magnification; adjustment of the lens-to-negative distance focuses the image on the enlarging paper on the baseboard. In enlargers that focus automatically these two adjustments are linked mechanically to keep the image sharp all the time. Enlargers are made in various sizes to take different maximum negative formats.

Printing papers (paper)
      Papers for enlarging and contact printing are produced in grades of differing exposure range—i.e., ratios of shortest to longest exposure to produce the lightest tone and a full black, respectively. The various grades yield prints of a normal tone range from negatives of different contrasts: a soft paper grade for a high-contrast negative, a normal paper for a normal negative, a hard paper for soft negatives, and so on. Paper grades are also numbered—typically from 0 to 5—in ascending order of contrast. Variable-contrast papers use a mixture of two emulsions of a different contrast and colour sensitivity; the contribution of each is controlled by filters in the path of the exposing light.

      Other characteristics of printing papers are the speed (slower for contact papers, faster for enlarging papers), image colour (blue-black to warm brown), surface texture (glossy, velvet, mat), and base thickness (single or double weight). Most printing materials use a resin-coated (plastic-laminated) paper base that absorbs no water during processing.

Printing exposures
      Correct printing exposures are determined by trial and error or by test strips given a series of progressively increasing exposures. More sophisticated exposure control systems measure either the brightness of selected image portions projected on the enlarger baseboard or the average light intensity reaching the paper during the exposure. Printing papers are exposed and processed in a darkroom lit by an olive-green or orange safelight. Printing papers are sensitive to violet, blue, and sometimes green light.

Print processing
      The processing of prints consists of development, an intermediate rinse or stop bath, fixing, and washing. The developer and fixer are similar in principle to those used for negative films. In the normal method, dish or tray processing, prints are immersed successively in the solutions in dishes laid out side by side. Development is checked visually, the print remaining in the developer until the image has reached its full density. For drying, the prints may be clipped to a line, placed in a heated print dryer, or squeegeed onto a mirror-finished plate for a high-gloss surface.

Stabilization processing
      Certain rapid-processing papers incorporate developing agents in their emulsions and are processed on a roller processor. This processor runs the paper through an activating bath for instant development and then through a stabilizing bath, followed by a pair of squeegeeing rollers from which the print emerges merely damp. This process takes about 10 to 15 seconds; the prints, however, do not keep quite as well as conventional prints, since unexposed silver salts are not removed from the emulsion but only converted into moderately light-stable compounds. Such prints can be made more permanent by subsequent fixing and washing.

Dry processing
      Processing baths can be completely eliminated by incorporating in the emulsion of the paper development and stabilization chemicals that become active on heating. One method is to disperse the processing chemicals in the emulsion in microscopic capsules containing the solution and a blowing agent. On passing the exposed paper over a heated roller, the blowing agent bursts the capsules, and the liberated processing solutions act on the silver halide immediately around each capsule. The liquid solvent instantly evaporates, leaving a dry print. Encapsulation materials are used for such purposes as making proof prints of negatives and reenlarging microfilm images. Certain non-silver processes in photocopying systems also offer dry processing.

Colour photography

Colour reproduction
      Present-day colour photographic processes are tricolour systems, reproducing different colours that occur in nature by suitable combinations of three primary-coloured stimuli. Each of these primary colours—blue-violet, green, and red—covers roughly one-third of the visible spectrum. Tricolour impressions can be produced by combining coloured lights (additive synthesis) or by passing white light through combinations of complementary filters, each of which holds back one of the primary colours (subtractive synthesis).

      In additive synthesis a combination of red and blue-violet light (e.g., light beams of the two colours directed on the same spot of a white screen) gives a purplish pink (magenta); equal parts of red and green produce yellow, and equal parts of green and blue-violet produce bluish green (cyan). Superimposition of all three light beams on a screen yields white; combinations of varying proportions of two or three of the colours produce virtually all the other hues.

      In subtractive synthesis yellow, magenta, and cyan filters or dye layers subtract varying proportions of the primary colours from white light. The yellow filter absorbs the blue component of white light and so controls the amount of blue present in a white-light beam that has passed through the filter. Similarly, the magenta filter controls the amount of green light left, and the cyan controls the amount of the red component. A cyan and a magenta filter superimposed in a white-light beam hold back both the red and the green component, making the emerging beam blue. Similarly, a cyan and a yellow filter together yield green, and a yellow and a magenta filter together yield red. Superimposing such filters or dye images of different densities in a white-light beam can therefore re-create any colour impression in the same way as superimposing light beams of the primary colours.

      The difference between additive and subtractive synthesis is the approach: in additive synthesis colours are built up by combining different intensities of primary-coloured light, and in subtractive synthesis colours are achieved by removing different proportions of primary-coloured light from white light. Most modern colour films are based on subtractive synthesis. Either method of colour synthesis should be capable of reproducing every existing colour in nature. In practice, the reproduction is imperfect; no filter dyes meet the required ideal specifications. Nevertheless, for most purposes reproduction is adequate.

Colour films
Reversal (slide) films
 To reproduce colour by subtractive three-colour synthesis (Figure 6—>), colour films first break down the colours of an image into their primary components by means of three separate sensitized layers, each of which responds exclusively to blue, green, or red light. The image in each layer is reversal-processed to yield a positive dye image in a colour complementary to the layer's spectral sensitivity. Thus, the blue-sensitive layer first yields a negative image of everything blue in the original scene (e.g., the blue sky) and then a positive image of everything that is not blue. This positive image is coloured yellow. Similarly, the green-recording layer yields a magenta positive image of everything that is not green, and the red-recording layer a positive cyan image of everything that is not red. Blue sky, for instance, does not figure in the yellow positive image but does figure in the magenta positive image (not being green) and in the cyan positive image (not being red). The magenta and cyan dyes in the areas that were blue sky are superimposed, and white light passing through the resulting transparency loses its green and red, but not its blue, component; thus, the sky appears blue. Similarly, green subject components end up as positive yellow image density in the blue-recording and positive cyan density in the red-recording layer, combining to green in the transparency. Yellow records as a negative image in the green-recording and red-recording layers, hence leaving a positive yellow image only in the blue-recording layer. All other colours are formed by similar combinations of different densities of the dye images.

Negative (print) films
      Negative colour materials work in a similar way but yield a negative dye image by direct development. Blue subject tones record in the blue-sensitive film layer to produce a yellow negative image. Green colour components yield a magenta dye image in the green-responding layer, and red components yield a cyan dye image in the red-recording layer. With respect to the subject, the colour negative therefore reverses the tones in brightness as well as in colour. Printing the colour negative on a colour paper with three differentially responding layers reverses the process once more, reconstituting the original subject colours in a positive print.

Colour-film structure
      Reversal colour film has these components: (1) A top layer of plain gelatin, which protects the underlying layers against abrasion and damage. (2) The first emulsion layer, which contains blue-sensitive silver halide plus a yellow-forming colour coupler. This is a colourless substance that reacts with the decomposition products of the developing agent to generate dye in all areas where a silver image is produced and in proportion to the density of that silver image. (3) A yellow filter layer, which holds back blue light from the subsequent emulsion layers. It disappears during the bleaching stage of processing. (4) The second emulsion layer, which contains blue- and green-sensitive silver halide plus a magenta-forming colour coupler. The blue sensitivity is suppressed by the yellow filter layer. (5) The next emulsion, which is blue-and-red sensitive (blue again being suppressed) and contains a cyan-forming colour coupler. (6) A substrate, which ensures optimum adhesion of the emulsion layers to the film base and may also contain light-absorbing silver to prevent the scattering of light by reflection from the support surface (halation). (7) The film base, or support, of clear cellulose acetate derivative (or sometimes polyester), typically about 0.005 inch thick. (8) The back of the support, which carries a light-absorbing layer (an alternative to the antihalation layer in the substrate); on roll film this also acts as an anticurl layer.

      This scheme can vary. Often one or more of the selectively sensitized layers is duplicated. Some reversal colour films do not incorporate colour couplers in the emulsions but introduce them in the processing solutions. Processing such nonsubstantive colour films is more complex than processing substantive-coupler films containing couplers in the emulsions. Negative colour films are similar, but the couplers are often already coloured yellow or red or both. The unreacted couplers remain as positive images, which compensate for some deviations of the image dyes from the colour characteristics of ideal dyes. Such negatives have an apparently overall reddish or orange tint.

      Some colour films come with different combinations of colour sensitivity and dye colour formed in the individual layers for deliberately falsified colour rendering (false-colour films) in special applications.

      Colour-positive (print) materials have a paper or white, opaque film base instead of transparent film and have no antihalation layer. The emulsion sequence may be different from the above scheme; their spectral sensitivities may be keyed to the transmission characteristics of the negative dyes for better colour reproduction.

      The dye images are reasonably lightfast but fade on prolonged exposure to ultraviolet-rich radiation. Colour transparencies and prints intended for continuous display may be protected by ultraviolet-absorbing coatings or filter layers.

Additive colour films
      These are simpler in structure and consist, in addition to protective and other interlayers, of a film base, carrying a filter raster, and a black-and-white emulsion layer. The raster consists of sequences of very narrow red, green, and blue transparent filter lines (up to 1,800 lines per inch) through which the light from the lens passes before it reaches the emulsion layer. The emulsion layer is processed to a positive image. Red subject portions cause silver to be deposited behind non-red (i.e., green and blue) filter elements, leaving the red filter lines transparent. Similarly, green subject details leave green filter lines transparent but block red and blue. Other colours affect areas behind two or even three filter lines—for example, yellow leaves red and green filter lines clear. In such areas the eye cannot resolve the separate filter elements but gets an additive impression of yellow. Other colours form corresponding additive effects, including white, where all three filter elements are transparent. Because of the presence of the filter elements everywhere in the image, additive colour transparencies are much denser than subtractive ones; at high magnification the filter raster pattern may also become visible. Additive colour transparencies are used only in rapid-access diffusion-transfer systems (see below Instant-picture photography (photography, technology of)).

Colour balance
      Colour film reacts to all hue and tone differences, including the prevailing light colour. A film recording approximately natural colours in daylight reproduces scenes photographed by tungsten light with a reddish overall tint—because this lighting is richer in red rays than is daylight. This spectral balance of different “white” light sources may be rated numerically by the colour temperature—a concept of theoretical physics that, with tungsten lighting, corresponds roughly to the absolute lamp-filament temperature. Such absolute temperatures are expressed in kelvins (kelvin) (K). The higher the colour temperature the richer the light is in bluish and the poorer it is in reddish rays and vice versa. Average daylight is rated at about 5,500 K, the light from an overcast sky from 6,500 K up; the colour temperature of tungsten lamps ranges between 2,600 and 3,400 K.

      To ensure correct “white-light” colour reproduction with different types of lighting, the sensitivities of the three film layers must be matched to the colour temperature of the light. Colour slide (reversal) films are therefore made in different versions balanced for faithful rendering either with 5,500 to 6,000 K light sources (such as daylight or electronic flash) or with specified tungsten lighting (3,200 to 3,400 K).

      Such accurate film balance matching is less vital with negative colour films since the colour rendering of the print can be modified during colour printing. “Universal” amateur negative colour films are usable with any light, from tungsten to daylight. For high quality, professional negative colour films are still preferentially balanced to either daylight or tungsten sources.

      Strongly coloured filters are suitable only for special effects; they overlay the colour image with the filter colour. Pale correction filters can match a film to a light source other than that for which it is balanced—e.g., pale blue, with a daylight-type film used in tungsten lighting, to raise in effect the colour temperature. Pale pink or amber filters similarly reduce the colour temperature for using artificial-light-balanced films in daylight. Colour-film manufacturers publish detailed recommendations of actual filters required for such conversion.

      In outdoor photography, especially involving distant views, an ultraviolet-absorbing filter is often required, as ultraviolet radiation records in the blue-sensitive layer of the film, producing an overall blue cast in the transparency. A pale pink skylight filter for outdoor subjects lit only by skylight counteracts the cold, bluish colour rendering resulting from such illumination.

Colour-film processing
      The processing sequence for colour materials is longer than for black-and-white films and requires more solutions. Development needs very precise timing and temperature control. Colour films can be processed in amateur developing tanks; professionals use sets of tanks in temperature-controlled water jackets with provision for standardized solution agitation.

Reversal colour-film processing
      Most colour films use a standard processing sequence and chemistry (usually available in kits). For substantive films (incorporating couplers in the emulsion) the sequence comprises: (1) development to form a negative silver image in each emulsion layer; (2) a reversal bath that renders developable the remaining silver halide in each emulsion layer; (3) colour development to produce a positive silver image in the remaining silver halide plus a coincident dye image by reaction with the colour couplers; (4) bleaching and fixing to reconvert the negative and positive silver images into silver halide and to dissolve the latter out of the emulsion, leaving only the three dye images; (5) a final rinse and stabilizer to remove soluble chemicals and improve light-fastness of the dyes; and (6) drying. There are also intermediate rinse stages. The complete sequence without drying takes a little longer than 30 minutes.

      Processing of nonsubstantive colour films, in which the couplers are in the colour developer, is more complex because each emulsion layer is reexposed by appropriately coloured light and colour-developed separately. This operation requires automated processing machinery.

Negative colour processing
      Negative colour films are practically all of the substantive-coupler type. Most again follow a standard processing sequence consisting of colour development (forming a negative silver image in each emulsion layer together with a corresponding dye image), a rinse, and a bleaching and fixing stage to convert the silver image into silver halide and dissolve that (plus residual halide) out of the emulsion. A final rinse and drying conclude the process, which, excluding drying, takes about 12 minutes. Substantially the same procedure is followed for processing positive colour papers.

Colour printing
      Colour print processing may be done in dishes or trays or in light-tight drums that are rotated manually or mechanically, processing solutions being poured in and out in succession. Professional colour laboratories use more elaborate versions of such rotating drum systems or roller or other automated machines that transport prints through the different solutions in turn.

Positive prints from colour negatives
      Positive prints may be obtained from colour negatives by enlarging the colour negative onto a positive colour paper. Colour control consists of modifying the colour of the printing light by yellow, magenta, and cyan filters (typically by inserting high-density filters of these colours to varying degrees in the light path) to obtain a print of the correct or desired colour balance. The light is thoroughly mixed in a diffusing box before reaching the negative. An alternative method is to have three light sources behind the yellow, magenta, and cyan filters and to adjust their relative intensities or switch them on for different exposure times.

      This subtractive, or white-light, printing method depends on subtracting or holding back colour components of white light. Commercial photofinishing printers often use an additive system in which prints are given successive exposures through high-density red, green, and blue filters. Each of these exposures forms the image in one of the emulsion layers of the paper; colour balance depends on the proportions of the individual exposures. In automatic colour printing systems the exposures are controlled by photocells that evaluate the red, green, and blue components of light transmitted by the negative.

Reversal colour printing
      Colour transparencies can be printed on a reversal colour paper similar to a reversal film and processed in an analogous way. The same kind of colour control with filters is again possible, but the colour effect of the subtractive filters or of the additive filter exposures is reversed.

Dye-destruction processes
      Dye-destruction processes differ from chromogenic colour materials (where colour images are produced during development) in starting off with emulsion layers containing the final dyes. During processing these are bleached in proportion to the silver image formed. Straightforward processing of a dye-destruction or dye-bleach material yields a positive image from a positive original and consists of: (1) development to form a silver image; (2) stop-fixing to arrest development and remove unexposed silver halide; (3) dye bleaching to bleach the dye in the areas containing a silver image; (4) silver bleaching to convert the silver image into silver halide; and (5) fixing to remove residual silver halide. Washing is done between all the processing stages.

      Obtaining a positive image from a negative requires a more elaborate processing sequence, analogous to reversal processing in a chromogenic system. Dye-bleach materials use far more light-stable dyes than those produced by colour-coupling development. The positive–positive procedure also yields duplicate transparencies on dye-bleach materials with a transparent film base.

Diffusion-transfer colour prints
      Materials derived from instant-picture diffusion-transfer processes (see below Instant-picture photography (photography, technology of)) have been adapted to colour print production. They are more expensive than traditional colour print materials but considerably easier to process. In their simplest form they require only a single highly alkaline activator bath followed by a water rinse, the whole sequence lasting about five to 10 minutes, with considerable processing latitude. Such materials exist for prints from either negatives or transparencies. The colour printing and filter control principles are the same as with the traditional processes described above.

Assembly colour prints
      The original method of producing colour prints was based on separation negatives obtained by photographing the original scene on separate black-and-white plates or films through a blue, green, and red filter, respectively. This method analyzes the subject in terms of its tricolour components in the same way as the initial negative images in a three-layer colour film. Positive prints from the separation negatives, converted into colour images (e.g., by toning) and superimposed on top of each other, yield a subtractive tricolour print.

      The main surviving assembly print process, dye transfer, uses a set of separation positives on a panchromatic matrix film made either from separation negatives of a colour transparency or by separation (three filtered exposures) from a colour negative. Appropriate processing converts the matrix film into a gelatin relief image whose depth is proportional to the positive silver density. Each matrix is soaked in a dye solution (yellow for the matrix derived from the blue-filter negative and so on), and the dyes from the matrices are transferred in succession to a single sheet of gelatin-coated paper. Elaborate care is required to ensure accurate superimposition (registration) of the dye images; the result is a positive colour print.

Transparency projection
      Many amateur colour pictures are in the form of transparencies, particularly on 35-mm film. These are usually mounted in plastic or card frames or bound between glass for projection on a screen in a darkened room. The projector consists of a lens, a holder for the slide, and a lighting system (lamp, reflector, and condenser lenses to concentrate the light onto the slide). Modern slide projectors take the slides in magazines or trays holding 30 to 50 or more slides. An automatic slide transport feeds each slide from the tray into the light path of the projector and may be operated from a remote control unit or by pulses from a tape recorder, which can also record a commentary to the complete slide series. Some projectors feature remote-controlled or automatic focusing to keep each successive slide image sharp on the screen.

      The standard miniature slide size is 2 × 2 inches for transparencies up to 15/8 × 15/8 inches; the most usual transparency format in such slides is 24 × 36 millimetres. Projectors for larger slides (e.g., 23/4 × 23/4 inches for transparencies up to 21/4 × 21/4 inches) and ultraminiature projectors (e.g., 10 × 14-millimetre transparencies in 3 × 3-centimetre slide frames) are suitably scaled-up or scaled-down versions of the standard models.

      The size of the projected image depends on the distance of the projector from the screen and the focal length of the projection lens. Projectors may also project from the back onto a translucent screen; such rear-projection setups are more compact, and the image is often bright enough for viewing in daylight. The rear-projection system is used in schools and for commercial displays. Elaborate slide shows are produced by linking two or more projectors aimed at the same or adjacent screen areas. With a suitably assembled slide set, the pictures can be made to change, overlap, and assemble, according to a predetermined program.

Instant-picture photography

History and evolution
      Cameras with built-in processing facilities, to reduce the delay between exposure and the availability of the processed picture, were proposed from the 1850s onward. The ferrotype process later adapted for “while-you-wait” photography by itinerant street and beach photographers goes back almost as far. Because of the messiness of handling liquid chemicals in or just outside the camera, such systems remained largely impractical. In the 1940s Edwin H. Land (Land, Edwin Herbert), a U.S. scientist and inventor, designed a film configuration that included a sealed pod containing processing chemicals in a viscous jelly or paste form to permit virtually dry processing inside the camera and yield a positive print within a minute or less of exposure. Land demonstrated (1947), and through his Polaroid Corporation marketed (1948), a camera and materials that realized this system. It used a positive sheet and negative emulsion, the latter being discarded after use. An instant-print colour film (Polacolor) was introduced in 1963 and an integral single-sheet colour film in 1972. After the mid-1970s other manufacturers offered similar instant-print processes. In 1977 Polaroid introduced an 8-mm colour movie film, and in 1982 it introduced still transparency films that permit rapid processing outside the camera.

Black-and-white diffusion transfer
      The Polaroid process is based on negative paper carrying a silver halide emulsion and a nonsensitized, positive sheet containing development nuclei. After the exposure the two sheets are brought into intimate contact by being pulled between a pair of pressure rollers. These rupture a sealed pod (attached to the positive sheet) to spread processing chemicals—in the form of a viscous jelly—between the two sheets. This reagent develops a negative image and causes the silver salts from the unexposed areas to diffuse into the positive layer and deposit metallic silver on the development nuclei. After about 30 seconds to one minute the negative and positive sheets are peeled apart and the negative can be discarded. In special versions of the process the negative may be washed and treated to give a conventional negative for normal enlarging.

      In the original Polaroid instant-picture process the material was a dual roll of negative and positive sheets. Later versions of this peel-apart process use film packs and sheet films. They require special cameras incorporating the pressure rollers thatoperate the spread of processing jelly while the peel-apart sandwich is fed out of the camera. Special camera backs with this mechanism allow the use of Polaroid materials in professional cameras taking interchangeable film holders or magazines. Peel-apart Polaroid systems include high-speed emulsions, high-contrast, process, transparency, and scientific materials.

      Silver diffusion-transfer processes were invented in 1939 in Belgium and Germany and were used for a number of years in office copying systems until superseded by dry copying processes.

The Polacolor process
      Polaroid colour film has a larger number of active layers, including a blue-sensitive silver halide emulsion backed by a layer consisting of a yellow dye–developer compound, a green-sensitive layer backed by a layer of magenta dye–developer, and a red-sensitive layer backed by a cyan dye–developer. The dye–developer in each case consists of dye molecules (not colour couplers) chemically linked to developing agent molecules.

      After exposure and activation by the alkaline jelly, the dye–developer molecules in each layer migrate into the adjacent silver halide layer. Development of exposed silver halide to a negative image anchors the dye–developer molecule in position. Dye–developer molecules in unexposed image areas are not used up by development but migrate into the receiving layer of the positive material. There they are immobilized, remaining as dye images corresponding to a positive of each silver halide layer in the negative film. The dyes thus re-create a full-colour positive image. The process depends on the controlled diffusion of the dye–developer molecules, achieved by spacing layers and balanced exposure and development time. Developing takes about one minute. Polacolor films include an 8 × 10-inch material for regular studio and view cameras (with separate processing machinery) and giant formats of 20 × 24 inches or even larger for special cameras.

Single-sheet process
      The Polaroid single-sheet, or integral, films contain all the negative and positive layers in a single preassembled film unit that is exposed through the transparent positive layer. The unit incorporates a viscous processing reagent that acts in principle similarly to the chemistry of the Polacolor process. It includes “opacifying” dyes and a highly opaque white pigment that together protect the negative layers against light during processing outside the camera. The pigment provides a background to the positive image after the dye–developer molecules from the negative layers have migrated into the receiving layer. Other constituents of the system neutralize residual active chemicals after processing, for all chemistry remains within the single-sheet print. The print size is about 31/2 × 41/4 inches, the effective image size about 31/8 × 31/8 inches. The Eastman Kodak and Fuji Photo Film companies also have marketed single-sheet films and cameras that accept each other's films. These materials and cameras are not compatible with the Polaroid products.

Autoprocess materials
      Because it requires cameras or camera backs with integral processing facilities, the instant-picture process is not suitable for conditions precluding immediate processing of the picture (e.g., in underwater or space photography), nor is it suitable for motion-picture or 35-mm cameras. Alternative procedures suggested to overcome this usually involve some form of semidry rapid-access processing. The Polaroid Autoprocess system uses 35-mm film in standard cartridges to fit any 35-mm camera. After exposure the film is driven through a tabletop processor, which sandwiches the film with a stripping film carrying a thin layer of processing fluid. The latter processes the negative image, causes the formation of a positive image by a diffusion-transfer process, and then releases the negative layers, which are finally removed from the film (together with residual chemicals) by the stripping material. The transparencies remaining on the 35-mm film are immediately ready for viewing and projection. Black-and-white as well as colour systems (by an additive process) are available in this form.

Applications
      Instant-picture processes have an advantage in applications that need quick access to a finished print. The initial field of the process was amateur snapshooting and instant portraits, from which evolved the taking of identification pictures for work and security passes. Such passes are made with special cameras that record a portrait together with personal details on a composite print that is then laminated to form a tamper-proof identity card. In studio photography instant prints provide a quick method of making exposure tests and checking the effect of lighting. Large- and giant-format Polacolor prints are used in studio portraiture; normal instant prints have numerous commercial applications. Instant pictures are also widely used in the laboratory to record experimental setups, for photomicrography and for infrared photography; for instant endoscopy and for clinical and forensic records; for rapid copying of normal colour transparencies; and for instant hard copy of oscilloscope, video, and computer graphic displays. Autoprocess transparencies are used for the rapid production of colour or black-and-white slides for lectures and publication and in various fields of scientific photography (including photomicrography) relying on the use of conventional 35-mm (usually single-lens reflex) cameras.

Special photosensitive systems
      The high working speed (efficiency of converting light into permanent images) of silver halides makes them almost the only materials suitable for camera use. Numerous light-sensitive systems not using silver have been known since the beginning of photography. In view of silver's high price, a number of substitute systems have grown in importance, and new ones have appeared. Most of them are limited to office copying, microfilming, the graphic arts, and other applications in which flat copy is reproduced.

      Electrophotography covers a number of processes that rely on photoconductive substances whose electrical resistance decreases when light falls on them. A layer of such a substance with a grounded backing plate is given a uniform electrostatic charge in the dark. When a light image is projected onto the surface, the photoconductor allows the electrostatic charge to leak away in proportion to the exposure. This leaves an “image” charge that can be converted, in various ways, into a visible image.

      In xerography the photoconductive layer is selenium, and the image is made visible by dusting the plate with an electrostatically charged powder (toner) having a charge that is the opposite of that of the electrostatic image. The powder adheres to the image portions only and is then transferred to a sheet of plain paper also under the influence of electrostatic fields. A final heat treatment fuses the powder into the paper for a permanent picture. The process usually makes a positive from a positive original. In office copying machines (the main application of xerography) the whole operating sequence is programmed and automated. A zinc oxide-coated paper may replace the selenium plate; if so, the pigment powder deposit is fused directly into the paper surface.

      The process is used mainly for line images without intermediate tones between black and white. Modified procedures permit continuous-tone reproduction and—with coloured pigments—also colour printing.

      In the electroplastic process a transparent thermoplastic serves as the photoconductive layer. After the plastic is charged and exposed, the residual electrostatic charge forms stresses in the thermoplastic. Controlled heating deforms the surface in the image areas into a grain pattern, which is frozen into the plastic on cooling. The resulting image is light-scattering and is viewed by reflection or in special projection systems.

Colloid and photopolymer processes
      A comparatively early non-silver process depended on organic colloid (gum or gelatin) treated with a bichromate. Exposure to light hardened the gelatin, rendering it insoluble, while unexposed portions could be washed away with warm water, leaving a relief image.

      Photopolymer systems substitute a plastic precursor in place of the gelatin. The plastic precursor polymerizes to an insoluble plastic when exposed to light, and the unexposed soluble material is washed out by a suitable solvent. Photopolymer processes have been adapted for forming resists (protective coatings) for etching, as, for instance, in the manufacture of printed circuits. In indirect photopolymer systems a light-sensitive substance is mixed with a plastic precursor and on exposure decomposes into compounds that initiate polymerization of the plastic. The polymerizable layer may include a pigment for a final coloured image. Superimposing colour images derived from separation negatives can yield positives; systems of this type are used for quick colour proofing in photomechanical reproduction.

Diazonium processes
      A diazo, or dyeline, process depends on the decomposition by light of organic diazonium salts. These salts can also couple with certain other compounds to form dyes. After exposure only the exposed (and decomposed) diazonium salt forms dye, producing a positive image from a positive original.

      The materials are usually papers or transparent supports impregnated with the required chemicals. They are mainly sensitive to ultraviolet rays and can therefore be handled by normal tungsten lighting.

      The light-decomposition of diazonium compounds also produces gaseous nitrogen. This phenomenon is utilized in vesicular processes that incorporate the diazonium compound in a thermoplastic layer. The nitrogen slowly diffuses out of this layer, but, if heat is applied immediately after exposure, the expanding nitrogen gas forms minute light-scattering bubbles visible as an image. The scattering power corresponds to the exposure. Further general exposure, after the plastic has cooled, decomposes the residual diazonium compound with gradual diffusion of the nitrogen out of the layer, destroying the latter's light sensitivity. This process and thermal dyeline systems are dry-processing instant-access systems and are used for making microfilm duplicates.

Photochromic systems
      Certain dyelike substances can exist in a colourless and a coloured state. They are called photochromic compounds. The coloured state is formed by exposure to radiations of a certain wavelength. The compound reverts to its colourless state either in the dark or on treatment with radiation of a different wavelength. This reversibility is a primary characteristic of photochromism, and it is an instant-image system involving no processing.

      Photochromic systems are used in microrecording (see below Microfilming and microreproduction (photography, technology of)). As the change of state takes place on a molecular level, the images are practically grain-free, and resolution is limited only by the resolving power of the optical system being used. Photochromic materials can be negative- or positive-working. With some photochromic compounds the dye image can be rendered permanent by optical or other treatment.

      Glasses containing certain metal compounds also act as photochromic materials. Exposure to light breaks down the compounds into metal that forms a visible (and permanent) image in the glass. Another type of photochromic glass contains silver halide crystals dispersed in the glass melt. The action of light decomposes the silver halide, forming a visible silver deposit. The halogen cannot escape from the glass, so it recombines with the silver in the dark and the image fades. Such photochromic glasses are incorporated in automatic light-control devices; light transmission decreases as the intensity of the light reaching the glass rises. Such glass has found use in certain types of sunglasses.

Electronic (electronics) photography
      As television cameras and recorders became more compact, home video recording began to replace home movies in the amateur field in the late 1970s. Video recording of still images was incidental to this; it became widely involved in the storage of computer-generated or computer-processed images on magnetic tape or discs, for instance, in satellite photography, radiography, image scanning in picture transmission, and photomechanical reproduction.

      A still video camera resembling traditional photographic apparatus (the Sony Mavica single-lens reflex) was first demonstrated in 1981. It uses a fast-rotating magnetic disc, two inches in diameter, recording on it up to 50 separate video images formed in a solid-state device in the camera. The images can be played back through a television receiver or monitor, or converted to paper in a printer that uses the video signals to control a printout device. Apart from being a potential rival to instant-picture photography, electronic records of this type are capable of direct transmission via telephone lines. Thus the process is of interest to press photographers, who can transmit pictures from their cameras directly to newspaper editorial offices without intermediate processing. The magnetic record also is able to directly control halftone engraving machines to engrave printing plates or cylinders.

Special techniques and applied photography

High-speed and stroboscopic photography
      High-speed photography is generally concerned with exposure times shorter than about 1/1,000 second (one millisecond) and often exposures shorter than 1/1,000,000 second (one microsecond). This field partly overlaps that of high-speed cinematography—sequences of very short exposures. Exposure times can be reduced by high-speed shutter systems or by short-duration flash sources.

      High-speed photography, together with high-speed cinematography, aids in the study of missiles, explosions, nuclear reactions, and other phenomena of military and scientific interest. In industry high-speed pictures show up movement phases of machinery, relays, and switches; dynamic fractures of materials or insulation breakdown; and, in natural science studies, flight movement of birds and insects.

High-speed shutters (shutter)
      The shortest exposure with mechanical shutters is about 1/4,000 second. Special high-speed shutter systems are magneto-optical, electro-optical, or electronic. A magneto-optical shutter (Faraday shutter) consists of a glass cylinder placed inside a magnetic coil between two crossed polarizing filters; so long as the filters remain crossed, virtually no light can pass through. A brief current pulse through the coil generates a magnetic field that rotates the light's plane of polarization in the cylinder so that during the pulse some light passes through the second polarizing filter. The electro-optical shutter (Kerr cell) is made up of a liquid cell of nitrobenzene fitted with electrodes and again placed between two crossed polarizers. An electric pulse applied to the electrodes changes the polarization properties of the nitrobenzene so that this arrangement again transmits light. Minimum exposure time is around five nanoseconds (5 × 10-9 second). Image converter tubes electronically transmit and amplify an optical image focused on one end of a tube onto a phosphorescent screen at the other end. Electrons flow in the tube only in the presence of an electric field, which can be controlled by short-time pulses down to a few nanoseconds.

High-speed light sources
      The shortest electronic-flash duration is around one microsecond. Spark discharges in air between electrodes yield still shorter exposures; discharge voltage may go up to tens or hundreds of thousands of volts. Short-duration pulses applied to X-ray tubes produce X-ray flashes for high-speed radiography. The shortest exposures are between 20 and 50 nanoseconds. Special switching modes turn lasers into high-speed sources with durations down to a fraction of a nanosecond.

      Generally the event photographed is made to trigger the exposure (the current pulse to operate the shutter or flash or spark source) to ensure correct synchronization. Examples are bullets interrupting a light beam to a photocell or self-luminous phenomena (explosions) triggering the system via a photocell circuit. The event and the exposure may be also triggered together by a signal from a common source.

Stroboscopic photography
      Electronic-flash units designed to flash in rapid succession (up to several hundred times a second) can photograph a moving subject in front of a stationary camera with its shutter open to yield multiple images of successive movement phases. The technique has been used in pictorial and sports photography (e.g., recording the movement of dancers or golfers) and for analyzing movement cycles without a motion-picture camera. Stroboscopic flash can be synchronized with a selected movement phase of an object in rapid cyclic motion (e.g., a rotating machine component); the moving component illuminated in this way then appears stationary.

      Photographs from airborne or spaceborne vehicles either provide information on ground features for military and other purposes (reconnaissance) or record the dimensional disposition of such features (surveying).

      Reconnaissance photographs call for maximum sharpness and detail rendering. Infrared films are often used to bring out details not discernible visually. In nonmilitary applications such photographs may reveal ecological factors (tree diseases, crop variations) and traces of archaeological sites not visible from the ground. Such shots are generally taken with cameras using 5- or 91/2-inch roll film in large magazines, built into the aircraft and operated electrically by the pilot or other crew member, or automatically at set intervals. Some systems incorporate a shutterless technique; the film runs continuously past a slit at a rate matched exactly to the image movement in the camera's focal plane as the aircraft flies over the ground (image motion compensation).

      Aerial survey is a systematic procedure of photographing the ground for map production; exposures are made at intervals to partly overlap the view of successive pictures. The individual photographs are enlarged to the same degree and then assembled in a precise mosaic. Aerial photographs taken under precisely specified conditions can serve for accurate measurements of ground details by stereoscopic evaluation (see below Stereoscopic and three-dimensional photography (photography, technology of)).

Satellite and space photography
      Satellites orbiting the Earth (Earth satellite) record changing meteorologic features (weather satellites) and broadcast the video images to ground stations where they may be recorded on magnetic tape or converted to hard-copy pictures by suitable printers. Video cameras in spacecraft sent to record surface details of other planets similarly scan electronically the view taken in by a lens and beam the scanning signals back to Earth, where they are recorded and reconverted to visible images. The signals are usually processed electronically to enhance image information and detail. Such enhancement often brings out more information than can be recorded by conventional photography. Similar techniques are used by military satellites monitoring ground features from high orbits above the Earth.

Underwater photography
      Underwater photography requires either special watertight cameras or pressure-resistant housings for normal cameras. In both cases camera functions are controlled through pressure-tight glands. A flat glass or plastic window is usually in front of the camera lens. The red and yellow absorption of the water more than a few feet below the surface turns colour photographs taken by daylight into virtually monochrome shots; hence artificial light is essential to show up the full colour range of fish and other underwater subjects. Light sources are battery-powered tungsten or tungsten-halogen lamps or electronic flash units (again in self-contained pressure-proof housings). For comfortable handling the weight of the housing with camera is adjusted to slight negative buoyancy. Complete camera and lighting outfits may be built into sledgelike or torpedo-like units with an electric or compressed-air motor for self-propulsion through the water.

      Since the refractive index ratio of glass to water is lower than for glass to air, the light-bending power of a glass lens is less in water than in air. This factor reduces the lens's angle of view and makes objects appear at about three-fourths of their actual distance. This difference must be allowed for in focusing—possibly by a suitably calibrated distance scale or by fitting the housing with a compensating porthole, which acts as a diverging lens.

      Underwater cameras with lenses designed for direct contact with the water eliminate the air space between the lens and the porthole. Such lenses can cover wider angles of view without distortion, but they do not give sharp images outside the water.

Close-range and large-scale photography
      Near photography to reveal fine texture and detail covers several ranges: (1) close-up photography at image scales between 0.1 and 1 (one-tenth to full natural size); (2) macrophotography between natural size and 10 to 20× magnification, using the camera lens on its own; (3) photomicrography at magnifications above about 20×, combining the camera with a microscope; and (4) electron micrography with an electron microscope at magnifications of 10,000 to 1,000,000×, which involves photography of the electron microscope's phosphor screen or placing a photographic emulsion inside the vacuum chamber of the electron microscope to record directly the image formed by the electron beams.

Close-up and macrophotography
      Supplementary close-up lenses or extension tubes (placed between the lens and camera body) allow the camera to focus on near distances for large scales of reproduction. Special close-up rangefinders or distance gauges establish exactly the correct camera-to-subject distance and precise framing of the subject field. Special simple close-up cameras, as in fingerprint recording and certain fields of medical photography, are permanently set to a fixed near distance and have a distance gauge or similar device built in. Screen-focusing cameras (view and single-lens reflex) need no such aids, as the finder screen shows the precise focus and framing.

      Extension tubes or extension bellows or both or “macro” lenses of extended focusing range are used for the macro range of distances. For optimum image quality macrophotographic lenses specially corrected for large image scales may be used or the camera lens reversed back to front.

      There are two principal methods of photographing through a microscope. In the first the camera, with its lens focused at infinity, is lined up in the optical axis of the microscope, which is also focused visually on infinity. In the other method the camera without lens is positioned behind the microscope eyepiece, which is focused to project the microscope image directly onto the film.

      Special photomicrographic cameras generally employ the second method. Microscope adapters to provide a light-tight and rigid connection between the camera and microscope are available for both systems. Such microadapters may incorporate their own shutter and a beam splitter system for viewing and focusing of the microscope image through a focusing telescope. Photomicrographs are the essential adjunct to all microscopy to record biologic, bacteriologic, physical, and other observations in black-and-white or colour.

Stereoscopic and three-dimensional photography
      Visual three-dimensional depth is perceived partly because of the fact that the human eyes see a scene from two viewpoints separated laterally by about 21/2 inches. The two views show slightly different spatial relationships between near and distant objects (parallax); the visual process fuses these stereoscopic views into a three-dimensional impression. A similar impression is obtained by viewing a pair of stereoscopic photographs taken with two cameras or a twin camera with lenses 21/2 inches apart, so that the left eye sees only the picture taken by the left-hand lens and the right eye only that of the right-hand lens. Binocular viewers or stereo-selective projection systems permit such viewing.

      Stereo photographs can also be combined in a single picture by splitting up the images into narrow vertical strips and interlacing them. On superimposing a carefully aligned lenticular grid on the composite picture, an observer directly sees all the strips belonging to the left-eye picture with the left eye and all the strips belonging to the right-eye picture with the right eye. Such parallax stereograms are seen in display advertising in shop windows. They also can be reproduced in print, overlaid by a lenticular pattern embossed in a plastic covering layer.

       photogrammetry makes use of stereo photography in measuring dimensions and shapes of ground objects in depth, as from successive exposure pairs made during an aerial survey flight. If all exposure parameters, including flying height, ground separation between exposures, and focal length of the aerial camera lens are known, the height of each ground feature can be measured. Photogrammetric plotting instruments do this and draw height contour curves of all features for aerial maps. Similar photogrammetric evaluation of stereo photographs of nearby subjects can also be made. For instance, it is possible to reconstruct accurately the scene of a highway accident. In industry a photogrammetric plot of an automobile model can be fed into a computer to program the machine tools that will shape the full-scale motor body components.

Infrared photography
      Images formed by infrared and heat radiations can be recorded directly, on films sensitive to them, or indirectly, by photographing the image produced by some other system registering infrared radiation.

      Silver halide emulsions can be sensitized to infrared rays with wavelengths up to around 1,200 nanometres (one nanometre is 1/1,000,000 of a millimetre). The usual sensitivity range is 800 to 1,000 nanometres. Direct infrared-recording aerial photography shows up ground features of differential infrared reflection but similar light reflection (e.g., different types of foliage) and cuts through haze and mist. Special colour films with an infrared-sensitive layer and processed to colours different from the natural rendering (false-colour films) show up such differences still more clearly. In forensic photography infrared pictures reveal ink alterations in forgeries, differentiate stains, and help to identify specific textiles and other materials. In medicine infrared photographs show subcutaneous blood vessels, as the skin is transparent to infrared.

      With suitable equipment it is possible to convert an infrared image into one visible on a fluorescent screen, where it can be photographed. In infrared scanner systems a moving mirror scans the object or scene and focuses the radiation onto an infrared-sensitive cell. The cell generates electric signals to modulate a light source, which, in turn, scans a photographic film or paper synchronously with the mirror. The resulting image records hotter and colder parts of the object as lighter and darker areas and can accurately establish actual temperatures of subject details. This system has been used to record temperature variations in the skin for the diagnosis of cancer.

Ultraviolet photography
      Invisible shortwave ultraviolet radiations can be recorded directly or used in fluorescence photography. For direct ultraviolet recording, the photographically useful wavelength range lies between 400 nanometres (visible violet) and about 200 nanometres and needs special optical systems transparent to ultraviolet rays (quartz, silica, or fluoride elements or combinations thereof). Light sources rich in ultraviolet such as mercury vapour lamps—with an ultraviolet-transmitting, but visually opaque, filter in front of the camera lens—ensure that the photograph records only the ultraviolet-reflecting characteristics of the subject.

      Fluorescence photography records the glow or visible light given off by certain substances when they are irradiated by ultraviolet rays. The object is illuminated by screening out the visible light with a filter that transmits only ultraviolet radiation, and another filter that absorbs the ultraviolet rays is placed over the camera lens, permitting only the visible light (fluorescence) to be recorded on the film. Normal lenses and panchromatic or colour materials are used.

      Ultraviolet photography can identify or separate pigments and fabrics and can detect forgeries of documents. Fluorescence photography can identify dyes, stains, specific chemical substances, and fluorescent components in microscope specimens. Ultraviolet microscopy offers increased resolution through the shorter-wavelength radiations employed. Aerial and satellite photography by ultraviolet can show up ultraviolet-reflective ground features.

Radiography and other radiation recording techniques
      Silver halide emulsions are sensitive to X rays, gamma rays, and charged particles emitted by radioactive substances. Some of these rays penetrate visually opaque materials to varying degrees to show up internal structures. Radiography covers techniques of recording the subsurface features of objects.

X-ray radiography
      X rays (X-ray) (wavelengths between 1/100 and 1/100,000 that of visible light) are produced by high-voltage electron streams bombarding an electrode in a vacuum tube. For radiography the object to be recorded is placed between an X-ray tube and the film; the film registers the differential absorption of the X rays by the object's internal structure as a projection shadowgraph.

      The most familiar application is in medicine for diagnosis and recording, including dental radiography. Industrial radiography permits nondestructive inspection of castings, welds, and engineering structures.

Gamma (gamma ray) radiography
      The technique of gamma-ray radiography is similar to that of X-ray radiography except that it relies on rays emitted by radioactive substances. Gamma rays have wavelengths from 100 to 1,000 times shorter than X rays and correspondingly greater penetrating power. Small gamma-ray sources are placed in areas inaccessible to X-ray tubes, such as inside pipelines. In all radiographic applications the exposure occurs under conditions of normal light, from which the radiographic film is protected by a light-tight (but radiation-transparent) wrapping.

      Autoradiography records the distribution of radioactive materials in botanical and histological specimens placed in contact with a photographic emulsion. This technique has been applied to the study of metabolism of plants and animals; it records the activity of organic compounds of radioactive isotopes introduced into the system of the plant or animal. In engineering studies autoradiography can be used to follow the transfer of radioactive substances from one surface to another in lubrication. The technique also has applications in machining and other metal-treatment processes.

Nuclear-track recording
      Tracks of subatomic particles, such as protons, electrons, and mesons, produced by nuclear reactions can be recorded by photographic means. The most common technique is to photograph the visible traces of such tracks in bubble or spark chambers with special camera and lens arrangements. Different arrangements can provide for coverage of large fields or the recording of tracks simultaneously from several directions for three-dimensional reconstruction.

      Particle tracks can be recorded directly in thick (up to one millimetre) emulsion layers or in emulsion stacks (up to 20 inches) carried in high-altitude balloons and in spacecraft and satellites. Special processing procedures are required to deal with these emulsion thicknesses.

Astronomical photography
      By the cumulative effect of light received over a long period, a photographic emulsion can record celestial objects too faint to be visible. Before radio telescopes (see telescope: Radio telescopes (telescope)), photography was the only way of detecting many such objects.

      Astronomical cameras are film- or plate-holding units built onto high-power telescopes, typically reflecting systems. The telescopes run on precision, clock-driven mounts to keep the optical axis stationary with respect to the sky area as the Earth rotates during an exposure time, which can run into several hours. For increased recording sensitivity, the telescope image may be intensified electronically.

      Astronomical photographs taken through narrow-band colour filters—including infrared or ultraviolet transmitting filters—show selective emission characteristics of stars. In the case of the Sun and of planets, such photographs can reveal some surface details not observable by white light. Colour photographs reveal colours not directly visible because the intensity of starlight is too low to stimulate the eye's colour-vision mechanism.

      Spectrography records the composition of light emitted by stars and other objects, the star image of the telescope being photographed through a diffraction grating, a device that disperses white light into constituent wavelengths. Elements present in the star or the gas mantle surrounding it can be identified from their characteristic spectral lines. Displacement of such lines from their known wavelength position can indicate the velocity with which the distant stellar systems recede from or approach the Earth.

Microfilming and microreproduction
      Microfilming is the copying of documents, drawings, and other such matter at a reduced scale—typically 1:15 to 1:42—for compact storage. Complete microreproduction systems include methods of filing the film copies for easy retrieval and reenlargement. Various duplication methods allow microfilm records to be extensively distributed.

      Documents, periodicals, and other printed matter are usually microfilmed on 16-mm film with an image size between 10 × 14 and 14 × 20 mm in a copying camera taking 100-foot lengths of film. Engineering drawings of high information content are microfilmed on 35-mm unperforated film with a standard image size of 32 × 45 mm. Films of up to 105 mm in width are also used. Automated microfilm cameras run continuously, documents being fed onto a moving band carried past the camera at a steady speed while the film runs past a slit at a matched rate.

      Readers and reader printers are desk-top projectors that display the frames reenlarged to about natural size on a back projection screen. In a reader printer the image may also be projected on sensitized paper for full-size enlargements. Advanced readers have elaborate retrieval systems based on frame coding and run the microfilm rolls through at high speed until a specific searched image is reached.

      Aperture cards or standard-size transparent jackets store microfilm images as single frames or groups of frames. Such unitized microfilms permit easier indexing and retrieval. Certain 35-mm microfilm cameras photograph the original document directly on film premounted in an aperture card and processed on the spot.

      Widely used is the unitized microfiche system, which carries up to 98 frames, each about 9 × 12 mm, on a 4 × 6-inch sheet of film. The microfiche camera repositions the film frame by frame after every exposure. Microfiche with a larger frame can also be produced by jacketing strips of 16-mm microfilm in multichannel plastic jackets 4 × 6 inches in size.

      For greater space saving, microfilm images may be reduced beyond 1:100 on high-resolution photochromic image materials. Extreme fine-grain silver copies then hold 3,000 to 4,000 individual frames on a single 4 × 6-inch film. This method, useful for complex catalogs and like purposes, offers easy retrieval of individual frames but requires a high-magnification reader.

The photography industry
      Present-day manufacture of cameras and other photographic equipment is concentrated in mass-production plants that make most of the components (camera bodies, lenses, shutters, and other parts) on largely automated machines; the components are then assembled by semiskilled or skilled labour. Smaller manufacturers of low- and medium-priced cameras obtain components for assembly from such specialist suppliers as shutter manufacturers and lens producers. High-quality precision cameras are produced on a smaller scale with automated fabrication of the engineering components but much more extensive manual assembly by highly skilled technicians. Components and functions of every camera are tested at every production stage; less expensive cameras are usually batch-tested by a sampling procedure.

      The raw material for lens manufacture covers a range of optical glasses (glass) of different optical characteristics. About 10 major worldwide glass producers supply the several dozen optical firms offering lenses of well-known brands. The glass is cast into blanks for specific lens elements and ground and polished to the required exact specifications, with the elements assembled in metal (sometimes plastic) mounts. Extensive production tests and optical performance checks safeguard quality standards.

      Film base is produced either by coating a solution of the base material on large drums, where it solidifies (film casting), or by extrusion of plastics, such as polyester, in film extruders. For print materials, paper of suitable purity is coated with a barium sulfate emulsion in gelatin, to provide a smooth white surface, and then with the silver halide emulsion. Silver halide emulsions are made by mixing silver nitrate with a solution of alkali halide—typically potassium bromide and iodide—in gelatin. The silver halide then precipitates out as fine crystals. After cooling to a jelly, shredding, and washing, the emulsion is remelted and treated to increase speed and contrast. Colour sensitizers (and colour couplers for colour emulsions) and additives are introduced, and the gelatin emulsion is machine-coated on wide continuous webs of paper or film. Generally several coatings are applied—up to a dozen for certain colour films. Operations from emulsion mixing onward are carried on in total darkness. After cooling and drying, the material is batch-tested for consistent characteristics and then is cut and packed.

      Photofinishing laboratories process most amateur and some professional photographers' films and prints. In the 1980s, virtually all of the total business of the laboratories in the United States was in colour processing.

      Photofinishing laboratories use machines that carry the films in spliced-together lengths or on racks through successive tanks of the processing solutions. Prints are usually made to standard formats on automatic enlargers, taking both the negatives and the paper in continuous rolls. The paper rolls of 250 or 500 feet are processed in continuous-strip processors, which deliver prints dry and ready for automatic cutting. Many printers have automatic exposure measurement based on overall negative density, with automatically controlled colour correction for colour negatives. High-capacity colour printers of this type can produce 2,000 to 3,000 prints per hour. Coding systems identify individual films and corresponding prints by customer or order number for final re-sorting. More exacting processing services grade colour negatives before printing by light transmission measurements through different colour filters; the resulting exposure data may be punched as edge codes in the film itself or programmed on perforated paper tape. When the tape is run through the printer together with the film, the perforations directly control the colour exposures and corrections. Advanced automatic printing systems may involve electronically controlled image enhancement.

      Enlargements to special sizes and colour printing for professional photographers require individual enlarging by skilled personnel on conventional enlargers (enlarger) with advanced automation features of focusing, exposure measurement, and colour control. Other processing services include duplication of transparencies, various types of photocopying (partly on coin-operated copiers in public places), microfilming, and microfilm processing.

L. Andrew Mannheim Ed.

Additional Reading
General works include The Focal Encyclopedia of Photography, rev. ed., 2 vol. (1965, reissued 1977); L.P. Clerc, Photography: Theory and Practice, rev. and enlarged ed. edited by D.A. Spencer, 4 vol. (1970–71; originally published in French, 1926), a classic treatise; The Theory of the Photographic Process, 4th ed. edited by T.H. James (1977), a classic work; C.B. Neblette, Neblette's Handbook of Photography and Reprography: Materials, Processes, and Systems, 7th ed. edited by John M. Sturge (1977); Encyclopedia of Practical Photography, 14 vol. (1977–79), edited by and published for the Eastman Kodak Company; John Hedgecoe, The Photographer's Handbook: A Complete Reference Manual of Techniques, Procedures, Equipment, and Styles, 2nd ed. rev. (1982); and Bruce Pinkard, The Photographer's Bible: An Encyclopedic Reference Manual (1983). See also Albert Boni (ed.), Photographic Literature (1962), and a supplemental volume, Photographic Literature, 1960–1970 (1972), an exhaustive and valuable bibliography; Wolfgang Baier, Quellendarstellungen zur Geschichte der Fotografie: A Source Book of Photographic History (1963, reissued 1977), detailed bibliographies and references to the literature on photographic developments, with an introduction in English; and Beaumont Newhall, Latent Image: The Discovery of Photography (1967, reissued 1983), helpful for the understanding of the establishment of the medium.The evolution of photographic techniques is traced in H. Fox Talbot (William Henry Fox Talbot), The Pencil of Nature (1844–46, reprinted 1969), the inventor's account, illustrated with 24 actual calotypes; Georges Potonniée, The History of the Discovery of Photography (1936, reissued 1973; originally published in French, 1925), a detailed account of the early days of photography; Josef Maria Eder, History of Photography (1945, reprinted 1978; originally published in German, 4th rev. ed., 2 vol., 1932), a pioneer Austrian work that deals primarily with the scientific and technological development of photography; Beaumont Newhall (ed.), On Photography: A Sourcebook of Photo History in Facsimile (1956), an anthology of the inventors' own accounts of various processes; D.B. Thomas, The First Negatives: An Account of the Discovery and Early Use of the Negative-Positive Photographic Process (1964); Joseph S. Friedman, The History of Color Photography, 2nd ed. (1968); Helmut Gernsheim, The History of Photography from the Camera Obscura to the Beginning of the Modern Era, 2nd ed. (1969), the first part of which was revised as The Origins of Photography (1982); and Gail Buckland, Fox Talbot and the Invention of Photography (1980).Camera history and technology is outlined in Leslie D. Stroebel, View Camera Technique, 5th ed. (1986), on the use of studio and field cameras in industrial, commercial, and other applications; Michel Auer, The Illustrated History of the Camera from 1839 to the Present, trans. from French and adapted by D.B. Tubbs (1975); Brian Coe, Cameras: From Daguerreotypes to Instant Pictures (1978); and Eaton S. Lothrop, Jr., A Century of Cameras from the Collection of the International Museum of Photography at George Eastman House, rev. and expanded ed. (1982).Lenses and optical principles are described in C.B. Neblette and Allen E. Murray, Photographic Lenses, rev. ed. (1973); Arthur Cox, Photographic Optics, 15th rev. ed. (1974), classic manual of lens principles and use; and Sidney F. Ray, The Photographic Lens (1979), an introduction.Film and the techniques of taking pictures are examined in Walter Nurnberg, Lighting for Photography: Means and Methods, 16th rev. ed. (1968, reissued 1971); and Michael Langford, Basic Photography, 5th ed. (1986), and Advanced Photography: A Grammar of Techniques, 4th ed. (1980), manuals of practical technique for professional photographers.Film processing and printing are the subject of D.H.O. John and G.T.J. Field, A Textbook of Photographic Chemistry (1963), basics of chemical reactions in black-and-white processing; C.I. Jacobson and R.E. Jacobson, Developing: The Negative-Technique, 18th ed. (1972), manual of all aspects of negative technique; C.I. Jacobson and L.A. Mannheim, Enlarging, 22nd ed. (1975), manual of positive technique in black and white and colour; L.F.A. Mason, Photographic Processing Chemistry, 2nd ed. (1975), detailed treatment of processing mechanisms and reactions; Grant Haist, Modern Photographic Processing, 2 vol. (1979), chemistry and technology of black-and-white and colour processing; and Jan Arnow, Handbook of Alternative Photographic Processes (1982).Colour photography is treated in Louis Walton Sipley, A Half Century of Color (1951); Ralph M. Evans, W.T. Hanson, Jr., and W. Lyle Brewer, Principles of Color Photography (1953), principles of colour rendering, response, and reproduction; D.A. Spencer, Colour Photography in Practice, rev. ed. by L.A. Mannheim and Viscount Hanworth (1966, reissued 1975), containing both theory and practical techniques; R.W.G. Hunt, The Reproduction of Colour, 3rd ed. (1975), a standard handbook on colour photography, television, and printing, with moderately advanced mathematical treatment; and Gert Koshofer, Farbfotographie, 3 vol. (1981), a complete historical review, including a lexicon of equipment and materials.Special photographic techniques and applications are the focus of Harold E. Edgerton and James R. Killian, Jr., Flash! Seeing the Unseen by Ultra High-Speed Photography, 2nd ed. (1954), and Moments of Vision: The Stroboscopic Revolution in Photography (1979, reprinted 1984); J. Bergner, E. Gelbke, and W. Mehliss, Practical Photomicrography (1966; originally published in German, 1961), a comprehensive manual; R.F. Saxe, High-Speed Photography (1966), a condensed but comprehensive survey; John Brackett Hersey (ed.), Deep-Sea Photography (1967); C.R. Arnold, P.J. Rolls, and J.C.J. Stewart, Applied Photography (1971), on scientific applications; H. Lou Gibson, Photography by Infrared, 3rd ed. (1978); and Gjon Mili, Gjon Mili: Photographs and Recollections (1980), on stroboscopic photography.Helmut Erich Robert Gernsheim L. Andrew Mannheim

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Universalium. 2010.

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