# Focal ratio (F-number)

## Description

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. For example, if a lens’s focal length is 10 mm and its entrance pupil diameter is 5 mm, the f-number is 2 and the aperture size would be expressed as f/2.

Related formulas## Variables

N | The f-number (dimensionless) |

f | Focal length (m) |

D | Diameter of the entrance pupil (effective aperture) (m) |