Definitions for f-numberˈɛfˌnʌm bər
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The ratio of the focal length of a lens to its diameter; a measure of the light-gathering ability of the lens.
In optics, the f-number of an optical system is the ratio of the lens's focal length to the diameter of the entrance pupil. It is a dimensionless number that is a quantitative measure of lens speed, and an important concept in photography.
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