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 can be calculated like this:

<math>\mbox{Aperture} \; \mbox{value} = {\frac{\mbox{Focal}\; \mbox{length}}{\mbox{Lens}\; \mbox{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 a direct relationship to the focal length. This is why terms like "effective focal length" are used. Make sure not to confuse aperture value with aperture. A large aperture opening gives a small aperture value, all cameras use aperture value on the controls and display so it is best to stick to that.

Exposure

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. It is the same as when you double the diameter of a lens you quadruple the light gathering area. A f/1.4 lens collects (5.6/1.4)² = 16 times the the amount of light of a f/5.6 lens.

Depth of field

Small aperture values will give a shallow depth of field where only the part that is exactly in focus will be sharp. Large aperture values will give a deep depth of field where most things in the picture are acceptably sharp independent of the distance from the camera.

Picture quality

Making a lens that has high quality at low aperture values is difficult and expensive so almost all lenses wil give lower quality at the lowest aperture values. At high aperture values there will be problems with diffraction and the pictures will start to lose sharpness. All lenses have a range of aperture values where they perform the best.