**Aperture** is the opening through which light is admitted into the camera and in photography it refers to the diameter of the aperture.

Some links on aperture:

Measurement of aperture is in **f-number** = the ratio of the focal length to the aperture diameter.

Calculation of f-number: where f is the focal length, and D is the diameter of the entrance pupil

- f/1 (big)
- f/1.4
- f/2
- f/2.8
- f/4
- f/5.6
- f/8
- f/11
- f/16
- f/22
- f/32
- f/45
- f/64
- f/90
- f/128 (small)