What is "f-number"

n. (context photography English) The ratio of the focal length of a lens to its diameter; a measure of the light-gathering ability of the lens.

In optics, the f-number (sometimes called focal ratio, f-ratio, f-stop, or relative aperture) of an optical system is the ratio of the lens's focal length to the diameter of the entrance pupil. It is a dimensionless number that is a quantitative measure of lens speed, and an important concept in photography. The number is commonly notated using a hooked f, i.e. N, where N is the f-number.

F-number or F number may refer to:

• f-number, the ratio of the lens's focal length to the diameter of the entrance pupil
• F number (chemistry), is a number that can be calculated for polycyclic aromatic hydrocarbons from their structure
• Fujita scale, a rating of a tornado's intensity by its impact on structures and vegetation