The pupil of the eye becomes bigger or smaller in order to control the amount of light it allows through. When a person is standing in a dark room, the pupil of his eye will be large to allow more light to enter. The camera also has a mechanism which controls the amount of light moving through the lens – the aperture.
The aperture controls not only the amount of light moving through the lens but also the depth of field. The f-stop is the unit in which the opening of the lens is measured. The f-stop number is inversely proportional to the size of the lens opening. This means f22 is a very small lens opening, with a large depth of field, and f4.5 has a wide lens opening with a shallow depth of field. With a wide lens opening and shallow depth of field, more light will reach the film. Thus a large lens opening is more suitable for photography in bad lighting conditions, but unfortunately with a loss in depth of field. Also see illustrations below. The f-stop number is equivalent to the diameter of the opening of the lens relative to the diameter of the front lens unit. An f-stop of f16 will therefore be 1/16 (one sixteenth) of the diameter of the front lens unit.
Tip: If you have a SLR camera with removable lenses remove the lens from your camera. Hold it in a position to enable you to look through it from the back. Now turn the f-stop ring on the lens from side to side. You will see the opening of the lens changing as you turn the ring.