relative aperture

American  

noun

Optics.

1. the ratio of the diameter of a lens, especially a camera lens, to the focal length; the reciprocal of the f number or focal ratio of the lens.

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relative aperture British  

noun

1. photog the ratio of the equivalent focal length of a lens to the effective aperture of the lens; written as f/n, f:n , or fn , where n is the numerical value of this ratio and is equivalent to the f-number

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Etymology

Origin of relative aperture

First recorded in 1885–90