Exposure compensation

Exposure compensation is a technique for adjusting the [[exposure (photography)|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 [[film 

speed#EI|exposure index]]), among other

factors.

Exposure compensation on still cameras

Snowy Mountains without exposure compensation

Same place with +2EV exposure compensation

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 (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 [[exposure value|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.

Adjustment for lighting distribution

The earliest reflected-light exposure meters were wide-angle, averaging types, measuring the average scene luminance. [[Light_meter#Exposure meter calibration|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

Main article: 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 (AEB)