The controlled presentation of a photosensitive surface to light in order to record an image. For a given sensitivity, exposure level is determined by the intensity of the light and the exposure duration, and must be correctly set to ensure satisfactory reproduction of tone and colour. In still cameras this is done by a suitable combination of lens aperture and shutter speed, but in motion-picture and video cameras the exposure time is normally fixed, and level is set by the lens aperture. An exposure meter is an instrument for measuring light, either incident on the subject or reflected from it, giving a scale reading (light value) from which lens aperture and shutter setting may be determined for a given photographic material. Modern still cameras embody a meter for reflected light, measuring through the lens, coupled for automatic exposure setting; while in video cameras a signal from the electronically scanned image automatically controls lens aperture.
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 exposure value (EV), with higher values denoting more light.
The "correct" exposure for a photograph is determined by the sensitivity of the medium used. 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 aperture.
An approximately correct exposure will be obtained on a sunny day using ISO 100 film, an aperture of f/16 and a shutter speed of 1/125th of a second.
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. Once the photographer has determined the exposure, aperture stops can be traded for halvings or doublings of speed, within limits.
The true characteristic of most photographic emulsions is not actually linear, (see sensitometry) 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.
The Zone System is another method of determining exposure and development combinations to achieve a greater tonality range over conventional methods by varying the contrast of the 'film' to fit the print contrast capability.
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.
Blown out highlights
In a photo the areas where information is lost, due to extreme brightness is known as "Blown out highlights", or "Flared highlights".
Film tends to have better latitude to cope with the highlight range, compared to digital, with a more gradual transition/tonal curve.
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