Exposure

In photography, exposure is the total amount of light allowed to fall on the photographic medium (Photographic film or image sensor) during the process of taking a photograph. Exposure is measured in lux seconds, and can be computed from exposed value (EV) and scene luminance.

The "correct" exposure for a photograph is determined by the sensitivity of the medium used. For photographic film, sensitivity is referred to as film speed and is measured on a scale published by the International organization of standardization(ISO). Faster film requires less exposure and has a higher ISO rating. Exposure is a combination of the length of time and the level of illumination received by the photosensitive material. Exposure time is controlled in a camera by shutter speed and the illumination level by the lens apture. Slower shutter speeds (exposing the medium for a longer period of time) and greater lens apertures (admitting more light) produce greater exposures.

An important principle of exposure is reciprocity. If one exposes the film or sensor for a longer period, a reciprocally smaller aperture is required to reduce the amount of light hitting the film to obtain the same exposure. For example, the photographer may prefer to make his sunny-16 shot at an aperture of f/5.6 (to obtain a shallow depth of field). As f/5.6 is 3 stops 'faster' than f/16, with each stop meaning double the amount of light, a new shutter speed of (1/125)/(2·2·2) = 1/1000 is needed. Once the photographer has determined the exposure, aperture stops can be traded for halvings or doublings of speed, within limits.

Cite:http://en.wikipedia.org/wiki/Exposure_%28photography%29