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relative aperture

noun, Optics.
1.
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.
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Based on the Random House Dictionary, © Random House, Inc. 2014.
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British Dictionary definitions for relative-aperture

relative aperture

noun
1.
(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
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition
© William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
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