from http://en.wikipedia.org/wiki/Exposure_compensation ---------------------------------------------------------- Exposure compensation using the Zone System[edit]
Main article: Zone System
An early application of exposure compensation was the Zone System developed by Ansel Adams and Fred Archer.[2] 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[# 5] 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. ------------------------------------------------------------
另外Zone system http://en.wikipedia.org/wiki/Zone_System