focal ratio

focal ratio
Optics., Photog.
f-number.
[1925-30]

* * *


Universalium. 2010.

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  • f ratio —    or f number or f stop (f/ or f)    a measure of the light gathering power of the lenses in cameras and telescopes.    The f ratio, for example f/4 or f4, is the aperture of the lens (the effective diameter of the lens, which may be reduced or… …   Dictionary of units of measurement

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  • F-number — In optics, the f number (sometimes called focal ratio, f ratio, or relative aperture [Smith, Warren Modern Lens Design 2005 McGraw Hill] ) of an optical system expresses the diameter of the entrance pupil in terms of the focal length of the lens; …   Wikipedia

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