Focal ratio is the ratio of the focal length (f ) of a mirror or lens to its diameter (D) expressed as a number (also called the f-number or f-stop), f / # =f/D. It is a measurement that describes the relative opening or aperture. For example, a lens with a 50-millimeter focal length with the aperture set at f/2 indicates an aperture diameter of 25 millimeters, whereas f/4 indicates an aperture of 12.5 millimeters.

Borrowing the language of photography, small focal ratios, below about f/6, are said to be fast and result in a brighter image for a given aperture. Large focal ratios, equal to or above about f/8, are said to be slow.