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stop number

 - 3 dictionary results

f-number

  [ef-nuhm-ber]
–noun Optics, Photography.
a number corresponding to the ratio of the focal length to the diameter of a lens system, esp. 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:
Also, f number.
Also called focal ratio, speed, stop number.

Origin:
1890–95
Dictionary.com Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2009.
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stop number

noun
the ratio of the focal length to the diameter of a (camera) lens system [syn: focal ratio] 
WordNet® 3.0, © 2006 by Princeton University.
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