- Exposure compensation
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 aless-than-optimal image. Factors considered may include unusual lighting distribution, variationswithin a camera system, filters, non-standard processing, or intended underexposureor overexposure. Cinematographers may also apply exposure compensation forchanges in shutter angleor film speed, among other factors.
Exposure compensation on still cameras
photography, some cameras include exposure compensation as a feature to allow theuser to adjust the automatically calculated exposure. Compensation can beeither positive (additional exposure) or negative (reduced exposure), and is commonly availablein third- or half-step [Photographers commonly refer to exposure changes in terms of “stops”, but properly, a 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.] increments, usually up to two or three steps in either direction; some digital cameras allow a greater range. Camera exposure compensation is commonly stated in terms of exposure value(EV); 1 EV is equal to one exposure step.
Exposure can be adjusted by changing either the lens
f-numberor 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.Zone System developed by Ansel Adamsand 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 whitefilm, the Zone System divided luminance [Zones refer to exposure; Adams (1981) 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.] 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 filmis slightly less than that of black and white film, and the tonal range of color reversal filmand 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.
Adams, Ansel. 1981. "The Negative". Boston: New York Graphic Society. ISBN 0-8212-1131-5
Wikimedia Foundation. 2010.