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f-number

[ef-nuhm-ber] /ˈɛfˌnʌm bər/
noun, Optics, Photography
1.
a number corresponding to the ratio of the focal length to the diameter of a lens system, especially a camera lens. In f /1.4, 1.4 is the f-number and signifies that the focal length of the lens is 1.4 times as great as the diameter. /, f/, f, f:
Abbreviation: f.
Also called f value, focal ratio, speed, stop number.
Origin of f-number
1890-1895
1890-95
Dictionary.com Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2015.
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British Dictionary definitions for f-number

f-number

noun
1.
(photog) the numerical value of the relative aperture. If the relative aperture is f8, 8 is the f-number and indicates that the focal length of the lens is 8 times the size of the lens aperture See also T-number
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition
© William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
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