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f-number

or f number

[ef-nuhm-ber]
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. Abbreviation: f/, f/, f, f:
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Compare relative aperture.

Origin of f-number

First recorded in 1890–95
Also called f value, focal ratio, speed, stop number.
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018
British Dictionary definitions for f-number

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 apertureSee also T-number
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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