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The ratio of the focal length of a lens or lens system to the effective diameter of its aperture. Also called f-stop.
(Photography) 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
a number corresponding to the ratio of the focal length to the diameter of a lens system, esp. a camera lens. Symbol: f/