Exposure compensation
Encyclopedia
Exposure compensation is a technique for adjusting the exposure
indicated by a photographic exposure meter, in consideration of factors that may cause the indicated exposure to result in a less-than-optimal image. Factors considered may include unusual lighting distribution, variations within a camera system, filters, non-standard processing, or intended underexposure or overexposure. Cinematographers may also apply exposure compensation for changes in shutter angle or film speed
(as exposure index), among other factors.
, some camera
s include exposure compensation as a feature to allow the user to adjust the automatically calculated exposure. Compensation can be either positive (additional exposure) or negative (reduced exposure), and is frequently available in third- or half-step, less commonly in full steps or even quarter-step increments, usually up to two or three steps in either direction; a few film and some digital cameras allow a greater range of up to four, five or even six steps in both directions. Camera exposure compensation is commonly stated in terms of EV
units; 1 EV is equal to one exposure step (or stop), corresponding to a doubling of exposure.
Exposure can be adjusted by changing either the lens f-number
or the exposure time; which one is changed usually depends on the camera's exposure mode
. If the mode is aperture priority
, exposure compensation changes the exposure time; if the mode is shutter priority
, the f-number is changed. If a flash is being used, some cameras will adjust it as well.
Many modern cameras incorporate metering systems that measure scene contrast as well as average luminance, and employ sophisticated algorithms to infer the appropriate exposure from these data. In scenes with very unusual lighting, however, these metering systems sometimes cannot match the judgment of a skilled photographer, so exposure compensation still may be needed.
developed by Ansel Adams
and Fred Archer. Although the Zone System has sometimes been regarded as complex, the basic concept is quite simple: render dark objects as dark and light objects as light, according to the photographer's visualization. Developed for black-and-white
film, the Zone System divided luminance into 11 zones, with Zone 0 representing pure black and Zone X representing pure white. The meter indication would place whatever was metered on Zone V, a medium gray. The tonal range of color negative film is slightly less than that of black-and-white film, and the tonal range of color reversal film and digital sensors even less; accordingly, there are fewer zones between pure black and pure white. The meter indication, however, remains Zone V.
The relationship between exposure compensation and exposure zones is straightforward: an exposure compensation of one EV is equal to a change of one zone; thus exposure compensation of −1 EV is equivalent to placement on Zone IV, and exposure compensation of +2 EV is equivalent to placement on Zone VII.
The Zone System is a very specialized form of exposure compensation, and is used most effectively when metering individual scene elements, such as a sunlit rock or the bark of a tree in shade. Many cameras incorporate narrow-angle spot meters to facilitate such measurements. Because of the limited tonal range, an exposure compensation range of ±2 EV is often sufficient for using the Zone System with color film and digital sensors.
Exposure (photography)
In photography, exposure is the total amount of light allowed to fall on the photographic medium during the process of taking a photograph. Exposure is measured in lux seconds, and can be computed from exposure value and scene luminance over a specified area.In photographic jargon, an exposure...
indicated by a photographic exposure meter, in consideration of factors that may cause the indicated exposure to result in a less-than-optimal image. Factors considered may include unusual lighting distribution, variations within a camera system, filters, non-standard processing, or intended underexposure or overexposure. Cinematographers may also apply exposure compensation for changes in shutter angle or film speed
Film speed
Film speed is the measure of a photographic film's sensitivity to light, determined by sensitometry and measured on various numerical scales, the most recent being the ISO system....
(as exposure index), among other factors.
Exposure compensation on still cameras
In photographyPhotography
Photography is the art, science and practice of creating durable images by recording light or other electromagnetic radiation, either electronically by means of an image sensor or chemically by means of a light-sensitive material such as photographic film...
, some camera
Camera
A camera is a device that records and stores images. These images may be still photographs or moving images such as videos or movies. The term camera comes from the camera obscura , an early mechanism for projecting images...
s include exposure compensation as a feature to allow the user to adjust the automatically calculated exposure. Compensation can be either positive (additional exposure) or negative (reduced exposure), and is frequently available in third- or half-step, less commonly in full steps or even quarter-step increments, usually up to two or three steps in either direction; a few film and some digital cameras allow a greater range of up to four, five or even six steps in both directions. Camera exposure compensation is commonly stated in terms of EV
Exposure value
In photography, exposure value denotes all combinations of a camera's shutter speed and relative aperture that give the same exposure. In an attempt to simplify choosing among combinations of equivalent camera settings, the concept was developed by the German shutter manufacturer in the 1950s...
units; 1 EV is equal to one exposure step (or stop), corresponding to a doubling of exposure.
Exposure can be adjusted by changing either the lens f-number
F-number
In optics, the f-number of an optical system expresses the diameter of the entrance pupil in terms of the focal length of the lens; in simpler terms, the f-number is the focal length divided by the "effective" aperture diameter...
or the exposure time; which one is changed usually depends on the camera's exposure mode
Mode dial
A mode dial or camera dial is a dial used on digital cameras to change the camera's mode. Most digital cameras, including dSLR and SLR-like cameras, support modes, selectable either by a dial or from a menu. On point-and-shoot cameras which support modes a range of scene types is offered. On dSLR...
. If the mode is aperture priority
Aperture priority
Aperture priority, often abbreviated A or Av on a camera mode dial, is a setting on some cameras that allows the user to choose a specific aperture value while the camera selects a shutter speed to match. The camera will ensure proper exposure...
, exposure compensation changes the exposure time; if the mode is shutter priority
Shutter priority
Shutter priority refers to a setting on some cameras that allows the user to choose a specific shutter speed while the camera adjusts the aperture to ensure correct exposure...
, the f-number is changed. If a flash is being used, some cameras will adjust it as well.
Adjustment for lighting distribution
The earliest reflected-light exposure meters were wide-angle, averaging types, measuring the average scene luminance. Exposure meter calibration was chosen to result in the “best” exposures for typical outdoor scenes; when measuring a single scene element (such as the side of a building in open shade), the indicated exposure is in the approximate middle of the film or electronic sensor's exposure range. When measuring a scene with atypical distribution of light and dark elements, or a single element that is lighter or darker than a middle tone, the indicated exposure may not be optimal. For example, a scene with predominantly light tones (e.g., a white horse) often will be underexposed, while a scene with predominantly dark tones (e.g., a black horse) often will be overexposed. That both scenes require the same exposure, regardless of the meter indication, becomes obvious from a scene that includes both a white horse and a black horse. A photographer usually can recognize the difference between a white horse and a black horse; a meter usually cannot. When metering a white horse, a photographer can apply exposure compensation so that the white horse is rendered as white.Many modern cameras incorporate metering systems that measure scene contrast as well as average luminance, and employ sophisticated algorithms to infer the appropriate exposure from these data. In scenes with very unusual lighting, however, these metering systems sometimes cannot match the judgment of a skilled photographer, so exposure compensation still may be needed.
Exposure compensation using the Zone System
An early application of exposure compensation was the Zone SystemZone system
The Zone System is a photographic technique for determining optimal film exposure and development, formulated by Ansel Adams and Fred Archer. Adams described how the Zone System was developed: "I take this opportunity to restate that the Zone System is not an invention of mine; it is a codification...
developed by Ansel Adams
Ansel Adams
Ansel Easton Adams was an American photographer and environmentalist, best known for his black-and-white photographs of the American West, especially in Yosemite National Park....
and Fred Archer. Although the Zone System has sometimes been regarded as complex, the basic concept is quite simple: render dark objects as dark and light objects as light, according to the photographer's visualization. Developed for black-and-white
Black-and-white
Black-and-white, often abbreviated B/W or B&W, is a term referring to a number of monochrome forms in visual arts.Black-and-white as a description is also something of a misnomer, for in addition to black and white, most of these media included varying shades of gray...
film, the Zone System divided luminance into 11 zones, with Zone 0 representing pure black and Zone X representing pure white. The meter indication would place whatever was metered on Zone V, a medium gray. The tonal range of color negative film is slightly less than that of black-and-white film, and the tonal range of color reversal film and digital sensors even less; accordingly, there are fewer zones between pure black and pure white. The meter indication, however, remains Zone V.
The relationship between exposure compensation and exposure zones is straightforward: an exposure compensation of one EV is equal to a change of one zone; thus exposure compensation of −1 EV is equivalent to placement on Zone IV, and exposure compensation of +2 EV is equivalent to placement on Zone VII.
The Zone System is a very specialized form of exposure compensation, and is used most effectively when metering individual scene elements, such as a sunlit rock or the bark of a tree in shade. Many cameras incorporate narrow-angle spot meters to facilitate such measurements. Because of the limited tonal range, an exposure compensation range of ±2 EV is often sufficient for using the Zone System with color film and digital sensors.
See also
- Exposure valueExposure valueIn photography, exposure value denotes all combinations of a camera's shutter speed and relative aperture that give the same exposure. In an attempt to simplify choosing among combinations of equivalent camera settings, the concept was developed by the German shutter manufacturer in the 1950s...
- Exposure index
- Light meterLight meterA light meter is a device used to measure the amount of light. In photography, a light meter is often used to determine the proper exposure for a photograph...
- Zone SystemZone systemThe Zone System is a photographic technique for determining optimal film exposure and development, formulated by Ansel Adams and Fred Archer. Adams described how the Zone System was developed: "I take this opportunity to restate that the Zone System is not an invention of mine; it is a codification...
- Exposure bracketing
- Auto Exposure Bracketing (AEB)