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Meaning of f-number

f-number

Pronunciation: (ef'num"bur), [key]
— n. Optics, Photog.
  1. 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. Abbr.: f/, f/, f, f: Also calledCf. relative aperture.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.