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.
focal length of a lens or mirror divided by its aperture. The smaller the focal ratio, the greater the speed of the optical system.
the ratio of the focal length to the diameter of a (camera) lens system
The ratio of the focal length to the aperture.
A ratio of the focal length and diameter of a mirror or lens.
the number produced when the focal length is divided by the lens diameter and is the ' F' number familiar to photographers. The smaller the ' F' number is, the brighter the image.
This is the ratio between the focal length of a telescope and the aperture. A telescope with an 8" aperture and 80" (2000mm) focal length has a focal ratio of f/10. Smaller focal ratios equate to shorter exposure times. An f/4 system is faster than an f/6 system, for example.
The relationship of a telescope's focal length to its aperture. An 80-mm refractor with a 900-mm focal length has a focal ratio of 11. This is written as f/11. Fast telescopes typically have focal ratios in the f/4 to f/6 range. Sometimes referred to as a richest field telescope (RFT), a fast telescope provides the widest possible field of view for a given aperture. Slow focal ratio telescopes usually fall in the f/8 to f/15 range and are sometimes called normal field telescopes (NFT).
This is found by dividing a telescope's focal length by its aperture. Any telescope with a focal ratio larger than f/12 would probably be said to be "slow", and any telescope with a focal ratio smaller than f/6, is said to be "fast".
The focal length of a telescope divided by the aperture.