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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.
As a general rule, a similarly correct exposure will be obtained by doubling the light reaching the film by opening the lens by one stop (in the above example to f/11) and halving the time the shutter opens (again, in the above example to 1/250th of a second). 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.
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