In photography, exposure is the total amount of light allowed to fall on the film during the process of taking a photograph. Exposure is measured in ev, with higher values denoting more light.
The correct exposure for a photograph is determined by the sensitivity of the film used. Film sensitivity is referred to as 'speed' and is measured on the ISO rating scale. Faster film requires less exposure and has a higher ISO rating. Exposure is controlled in a camera by shutter speed and lens aperture. Slower shutter speeds and greater lens apertures produce greater exposures.
An approximately correct exposure will be obtained on a sunny day using ISO 125 film, an aperture of f/16 and a shutter speed of 1/125th of a second. This is called the sunny f/16 rule.
An important principle of exposure is reciprocity. If one exposes the film 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, he can trade aperture stops for halvings or doublings of speed, within the bounds of reciprocity failure, described below. In digital cameras, the photographer can also trade the ISO number against speed and aperture in any combination that conserves reciprocity.
The actual characteristic of most photographic emulsions is not actually linear, but it is close enough over the exposure range of about one second to 1/1000th of a second. Outside of this range, it becomes necessary to increase the exposure from the calculated value to account for this characteristic of the emulsion. This characteristic is known as reciprocity failure. The film manufacturer's data sheets should be consulted to arrive at the correction required as different emulsions have different characteristics.
The Zone System is another method of determining exposure.
Today, most cameras automatically determine the correct exposure at the time of taking a photograph by using a built-in light meter, or multiple point meters interepreted by a built-in computer, see metering mode.
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