The ratio of the focal length of a lens or mirror to its diameter.
The ratio of the focal length (F) of a mirror or lens to its diameter (D) expressed as a number, f/# = F/D. 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...
The focal length of a telescope divided by its aperture (opening) or primary mirror diameter.
This is the relationship between the diameter of a lens and its focal length. It describes the 'speed' of the lens. For example, a lens with a focal length of 60cm and a diameter of 4cm has a focal ratio of f/15, because 60cm divided by 4cm is 15
(f/number) A lens or mirrors focal length divided by its aperture. For instance, a telescope with an 80-mm-wide lens and a 400-mm focal length has a focal ratio of f/5.
[n] - the ratio of the focal length to the diameter of a (camera) lens system