focal ratio(n = noun.linkdef) f number, speed, stop number - the ratio of the focal length to the diameter of a (camera) lens system;
is a kind of ratio

aspect ratio, case-to-infection ratio, focal, focal distance, focal epilepsy, focal infection, focal length, focal point, focal seizure, loss ratio, output-to-input ratio, p/e ratio, price-to-earnings ratio, ratio, signal-to-noise ratio, signal/noise ratio