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 exposure value (EV) and scene luminance over a specified area.
In photographic jargon, an exposure generally refers to a single shutter cycle. For example: a long exposure refers to a single, protracted shutter cycle to capture enough low-intensity light, whereas a multiple exposure involves a series of relatively brief shutter cycles; effectively layering a series of photographs in one image. For the same film speed, the accumulated photometric exposure (H) should be similar in both cases.
“Correct” exposure may be defined as an exposure that achieves the effect the photographer intended[6]. The purpose of exposure adjustment (in combination with lighting adjustment) is to control the amount of light from the subject that is allowed to fall on the film, so that it falls into an appropriate region of the film’s characteristic curve and yields a “correct” or acceptable exposure.
Overexposure and underexposure
White chair: Deliberate use of overexposure for aesthetic purposes
A photograph may be described as overexposed when it has a loss of highlight detail, that is, when the bright parts of an image are effectively all white, known as “blown out highlights” (or “clipped whites”). A photograph may be described as underexposed when it has a loss of shadow detail, that is, the dark areas indistinguishable from black, known as “blocked up shadows” (or sometimes “crushed shadows,” “crushed blacks,” or “clipped blacks,” especially in video).
Manual exposure
In manual mode, the photographer adjusts the lens aperture and/or shutter speed to achieve the desired exposure. Many photographers need to control aperture and shutter independently because opening up the aperture increases exposure, but also decreases the depth of field, and a slower shutter increases exposure but also increases the opportunity for motion blur.
‘Manual’ exposure calculations may be based on some method of light metering with a working knowledge of exposure values, the APEX system and/or the zone system.
Automatic exposure
A camera in automatic exposure (AE) mode automatically calculates and adjusts exposure settings in order to match (as closely as possible) the subject’s mid-tone to the mid-tone of the photograph. For most cameras this means using an on-board TTL exposure meter.
Aperture priority mode gives the photographer manual control of the aperture, whilst the camera automatically adjusts the shutter speed to achieve the exposure specified by the TTL meter. Shutter priority mode gives manual shutter control, with automatic aperture compensation. In each case, the actual exposure level is still determined by the camera’s exposure meter.
Exposure compensation
Main article: exposure compensation
The purpose of an exposure meter is to estimate the subject’s mid-tone luminance and indicate the camera exposure settings required to record this as a mid-tone. In order to do this it has to make a number of assumptions which, under certain circumstances, will be wrong. If the exposure settings indicated by an exposure meter are be taken as the “reference” exposure, the photographer may need to deliberately overexpose or underexpose in order to compensate for known or anticipated metering inaccuracies (see exposure meter).
Cameras with any kind of internal exposure meter usually feature an exposure compensation setting which is intended to allow the photographer to simply offset the exposure level from internal meter’s estimate of subject mid-tone. Exposure compensation is commonly calibrated in exposure values, where +1EV means 1 stop overexposed and −1EV means 1 stop underexposed.
Exposure compensation is particularly useful in combination with Auto exposure mode, as it allows the photographer to bias the exposure level without resorting to full manual exposure and losing the flexibility of auto exposure. On low-end video camcorders, exposure compensation may be the only manual exposure control available.
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