f-number - Notation

A numerical system of indicating the size of the aperture stop in a camera lens, which determines how much light is transmitted to the film and hence the control of exposure in conjunction with shutter speed. The f-number is calculated by dividing the focal length of the lens by the diameter of the clear aperture of the lens as given by the iris diaphragm. A small f-number indicates a large aperture. A lens of focal length 100 mm with apertures of 50 mm and 12·5 mm will have f-numbers of f/2 and f/8 respectively. Increasing the f-number by ‘stopping-down’ the iris reduces the light transmitted and increases depth of field. The usual range of numbers is from f/1·4 to f/32 in increments that give a progressive halving of transmission by a series such as 1·4, 2, 2·8, 4, 5·6, 8, 11, 16, 22, and 32.

The initial letter is shown capitalized due to technical restrictions.

In optics, the f-number (sometimes called focal ratio, f-ratio, or relative aperture) of an optical system expresses the diameter of the entrance pupil in terms of the effective focal length of the lens.

Notation

The f-number f/#, often notated as N, is given by