noun \ˈef-ˌnəm-bər\

Definition of F-NUMBER

:  the ratio of the focal length to the aperture in an optical system
:  a number following the symbol f/ that expresses the effectiveness of the aperture of a camera lens in relation to brightness of image so that the smaller the number the brighter the image and therefore the shorter the exposure required

Origin of F-NUMBER

focal length
First Known Use: circa 1903


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