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, among other factors.
Most DSLR cameras have a display whereby the photographer can set the camera to
either over or under expose the subject by up to three f-stops in 1/3rd stop
intervals. Each number on the scale represents one f-stop, decreasing the
exposure by one f-stop will halve the amount of light reaching the sensor. The
dots in between the numbers represent 1/3rd of an f-stop.
Exposure compensation on still cameras In photography, some cameras include
exposure compensation as a feature to allow the user to adjust the
automatically calculated exposure. Compensation can be either positive or
negative, 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, 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.
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, 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 often will be underexposed, while a scene with
predominantly dark tones 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 System
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.
See also Exposure value
Exposure index Light meter
Zone System Exposure bracketing
Auto Exposure Bracketing Notes
^ a b c By default, the Minolta 7000 and 9000 support exposure-compensation in
half-step increments over a range of ±4.0 EV, however, in conjunction with
the Minolta Program Back Super 70 / 90 or the 100-Exposure Back EB-90
quarter-steps are supported over an effective range of ±6.0 EV. In order to
cope with the finer granularity, aperture and shutter speed settings are
displayed in a proprietary suffixed notation, that is, a full f-stop of 2.8
is displayed as 2.80, the next quarter-steps would be 2.81, 2.82, 2.83,
before it would continue with 4.00, etc. ^ Photographers commonly refer to
exposure changes in terms of "stops", but properly, an aperture stop is a
device that regulates the amount of light, while a step is a division of a
scale. The standard exposure scale consists of power-of-two steps; a
one-step exposure increase doubles the exposure, while a one-step decrease
halves the exposure; these steps are what are commonly referred to as stops.
^ The Nikon F5 and F6 support an exposure-compensation range of ±5.0 EV.
^ With Firmware 2.0, the Sony Alpha DSLR-A850 and DSLR-A900 support an
extended exposure-compensation range of ±5.0 EV.
^ Zones refer to exposure; Adams distinguishes among exposure zones,
negative density values, and print values. The negative density value is
controlled by exposure and the negative development; the print value is
controlled by the negative density value, and the paper exposure and
development. References
^ Exposure Compensation. "By Geoff Lawrence"
^ van der Walt, Ed.. “ISO and Film Speed”. Under Basic Photography.
Illustrated Photography.com. Retrieved 7 July 2011.
^ Adams, Ansel. The Negative. Boston: New York Graphic Society. ISBN
0-8212-1131-5