Aperture value

The aperture value (f-number) is a dimensionless value that expresses the diameter of the effective aperture (entrance pupil) in terms of the effective focal length of the lens.

For a single lens element it is calculated like this:

<math>Aperture \; value = {\frac{Focal\; length}{Lens\; diameter}} </math>

A modern camera lens is a very complex design that uses many lens elements where the lens diameter and aperture diameter may not have direct relationship to the focal length. This is why terms like "effective focal length" are used.

The aperture value is a measure of how much light is entering the camera over a given amount of time. If you halve the aperture value you quadruple the amount of light entering the camera.