Exposure Value (EV)
Exposure Value (EV) represents a ratio of all possible combinations of film speed and aperture settings that can render any given exposure. While the film speed will determine the degree to which the subject appears to be in motion, the aperture setting will influence the depth of field within the photographed scene.
In the 1950s, a formal equation including these variables was derived to figure out exposure value. In addition to the film speed and aperture setting, the equation for exposure value also includes the shutter speed, the luminance of the setting and a calibration constant.
Photographers serious about capturing the proper exposure for a scene should calculate the exposure value appropriate for the shot. While a photographer can sit down and work out the values of the equation, it is easier to use a light meter that calculates the exposure value of the scene.
For photographers interested in incorporating more dimensions in the exposure values, the Additive system Photographic Exposure (APEX) is a more comprehensive measurement that also takes the shutter markings into account.