# focal ratio

Diagram of the focal ratio of a simple optical system where *f* is the focal length and *D* is the diameter of the objective.

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.