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.