relative aperture

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.

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