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Exposure (photography)
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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.
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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.
Photometric and radiometric exposure
Photometric or luminous exposure is the accumulated physical quantity of visible light energy applied to a surface during a given exposure time, wavelength weighted by the luminosity function. It is defined by:
where
- is the luminous exposure (usually in lux seconds)
- is the image-plane illuminance (usually in lux)
- is the exposure time (seconds)
The radiometric quantity radiant exposure is sometimes used instead; it is the product of image-plane irradiance and time, the accumulated amount of incident light energy per area. 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). 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 is conventionally expressed as log10(H). Photographers more familiar with base-2 logarithmic scales (such as exposure values) can convert using .
Exposure settings
"Correct" exposure may be defined as an exposure that achieves the effect the photographer intended. 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
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
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 underexposed and -1EV means 1 stop overexposed.
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.
Exposure time
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 and a shutter speed of 1/100th of a second. This is called the sunny 16 rule: at an aperture of 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.
Reciprocity
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. For example, the photographer may prefer to make his sunny-16 shot at an aperture of (to obtain a shallow depth of field). As is 3 stops "faster" than , with each stop meaning double the amount of light, a new shutter speed of (1/125)/(2) = 1/1000 is needed. 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. 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.
Digital camera image sensors can also be subject to a form of reciprocity failure.
Determining exposure
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. Digital cameras can achieve similar results (high dynamic range) by combining several different exposures (varying only the shutter speeds) made in quick succession.
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 interpreted by a built-in computer, see metering mode.
Negative/Print film tends to bias for exposing for the shadow areas (film dislikes being starved of light), with digital favouring exposure for highlights. See latitude below.
Latitude
Latitude is the degree by which one can over, or under expose an image, and still recover an acceptable level of quality from an exposure. Typically negative film has a better ability to record a range of brightness than slide/transparency film or digital. Digital should be considered to be the reverse of print film, with a good latitude in the shadow range, and a narrow one in the highlight area; in contrast to film's large highlight latitude, and narrow shadow latitude. Slide/Transparency film has a narrow latitude in both highlight and shadow areas, requiring greater exposure accuracy.
Negative film's latitude increases somewhat with high ISO material, in contrast digital tends to narrow on latitude with high ISO settings.
Highlights
Areas of a photo where information is lost due to extreme brightness are described as having "blown-out highlights" or "flared highlights".
In digital images this information loss is often irreversible, though small problems can be made less noticeable using photo manipulation software. Recording to RAW format can ameliorate this problem to some degree, as can using a digital camera with a better sensor.
Film can often have areas of extreme overexposure but still record detail in those areas. This information is usually somewhat recoverable when printing or transferring to digital.
A loss of highlights in a photograph is usually undesirable, but in some cases can be considered to "enhance" appeal. Examples include black-and-white photography and portraits with an out-of-focus background.
Blacks
Areas of a photo where information is lost due to extreme darkness are described as "crushed blacks". Digital capture tends to be more tolerant of underexposure, allowing better recovery of shadow detail, than same-ISO negative print film.
Crushed blacks cause loss of detail, but can be used for artistic effect.
See also
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