noun, Optics.
the ratio of the diameter of a lens, especially a camera lens, to the focal length; the reciprocal of the f number or focal ratio of the lens.
relative aperture
(photog) the ratio of the equivalent focal length of a lens to the effective aperture of the lens; written as f/n, f:n, or fn, where n is the numerical value of this ratio and is equivalent to the f-number


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