Over the next few weeks I will explain some of the terms often used in photography starting with the basics and getting on to some of the more esoteric terms later on.
Today we will start with exposure. Exposure is the amount of light received by the film or sensor.This is determined by your aperture, shutter speed and the sensitivity (ISO) of your film or sensor.
The first of these variables is aperture. The aperture refers to the size of the opening in the lens that controls the intensity of the light that hits the sensor and is controlled by a series of overlapping blades that work much like our pupil. It also controls the depth of field.
Successive apertures halve the amount of incoming light. To achieve this, the diaphragm reduces the aperture diameter by a factor 1.4 (square root of 2) so that the aperture surface is halved each successive step as shown on this diagram.
Because of basic optical principles, the absolute aperture sizes and diameters depend on the focal length. For instance, a 25mm aperture diameter on a 100mm lens has the same effect as a 50mm aperture diameter on a 200mm lens. If you divide the aperture diameter by the focal length, you will arrive at 1/4 in both cases, independent of the focal length. Expressing apertures as fractions of the focal length is more practical for photographers than using absolute aperture sizes. These "relative apertures" are called f-numbers or f-stops. On the lens barrel, the above 1/4 is written as f/4 or F4 or 1:4.
It is important to remember that f-numbers are fractions of the focal length, "higher" f-numbers represent smaller apertures.