F-number meaning

ĕf'nŭm'bər
The ratio of the focal length of a lens or lens system to the effective diameter of its aperture.
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A number that represents the relative aperture of a lens, equal to the focal length divided by the effective diameter of the lens aperture: a higher number indicates a smaller opening.
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See f-stop.
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(photography) The ratio of the focal length of a lens to its diameter; a measure of the light-gathering ability of the lens.
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Origin of f-number

f(ocal length) number