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 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.

The radiometric quantity radiant exposure[3] is sometimes used instead; it is the product of image-plane irradiance and time, the accumulated amount of incident light energy per area.[4] If the measurement is adjusted to account only for light that reacts with the photo-sensitive surface, that is, weighted by the appropriate spectral sensitivity, the exposure is still measured in radiometric units (joules per square meter), rather than photometric units (weighted by the nominal sensitivity of the human eye).[5] Only in this appropriately weighted case does the H measure the effective amount of light falling on the film, such that the characteristic curve will be correct independent of the spectrum of the light.

Many photographic materials are also sensitive to "invisible" light, which can be a nuisance (see UV filter and IR filter), or a benefit (see Infrared photography and Full spectrum photography). The use of radiometric units is appropriate to characterize such sensitivity to invisible light.

In sensitometric data, such as characteristic curves, the log exposure[2] is conventionally expressed as log10(H). Photographers more familiar with base-2 logarithmic scales (such as exposure values) can convert using 3.32 log2(H) ˜ log10(H).
[edit] Exposure settings

"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.
[edit] 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).[7][8][9] As the image to the right shows, these terms are technical ones rather than artistic judgments; an overexposed or underexposed image may be "correct", in that it provides the effect that the photographer intended.
[edit] 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.
[edit] 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.
[edit] 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 setting indicated by an exposure meter is taken as the "reference" exposure, the photographer may wish 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 the internal meter's estimate of appropriate exposure. Frequently calibrated in stops,[10] also known as EV units,[11] a "+1" exposure compensation setting indicates one stop more (twice as much) exposure and "–1" means one stop less (half as much) exposure.[12][13]

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.
[edit] Exposure time
Main article: shutter speed
A two second exposure of a fire poi ball dance

The 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 for 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 aperture. Slower shutter speeds (exposing the medium for a longer period of time) and greater lens apertures (admitting more light) produce greater exposures.

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 1/100th of a second. This is called the sunny 16 rule: at an aperture of f/16 on a sunny day, a suitable shutter speed will be one over the film speed (or closest equivalent).

A scene can be exposed in many ways, depending on the desired effect a photographer wishes to convey.