The ratio of the focal length divided by the diameter of the primary mirror or lens.
The effective focal length of an optical system divided by the diameter of the primary optical component.
The ratio of the focal length (F) of a mirror or lens to its diameter (D) expressed as a number; f/# = F/D. Also defines the cone angle of the beam. Small focal ratios e.g. f/# = F/D = 1 are said to be “fast” and represent a very large cone angle. Large focal ratios e.g. f/# = 35 are said to be “slow” and indicate a very small cone angle.