Digital Photography Technique: Camera 3rd basic mechanism is the diaphram

A camera's third basic mechanism is the diaphragm, which allows us to regulate the intensity of the beam of light which passes through the lens and onto the film or digital sensor. The diaphragm is very similar to the iris of the eye, a muscle that contracts or dilates to regulate the diameter of the pupil and therefore the intensity of light that reaches the retina.

The first diaphragms were metallic discs with holes in the middle. A photographer would chose a disc with the proper aperture and place it in front of the lens. Nowadays the diaphragm is a variable aperture mechanism placed inside the lens. It's made of small metal plates which, depending on the selected position, make the diaphragm's aperture larger or smaller.

The diaphragm's aperture is regulates with it's corresponding selector ring, which is the third essential camera control. On the aperture scale, each one is identified by a number ( f-stop ). The scale is ordered so that each aperture transmits an intensity half of the previous and twice of the following . On most machines you will find a scale similar to the following, although the maximum and minimum apertures vary from one lens to another: From now on we will abbreviate "diaphragm aperture" with it's technical expression, "f", because it is necessary to distinguish f8, f4 and f2 from speeds 8, 4 and 2 (1/8, 1/4 and 1/2 second).

It is important to realize that the smaller the number, the greater the amount of light that passes, and for every stop to the right the intensity gets cut in half. (For example, f11 lets half as much light through as f8, four times less than f5.6, eight times less than f4 and twice that of f16, etc.)