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Zone System
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=== Effective film speed === The ISO standard for black-and-white negative film, ISO 6:1993, specifies development criteria that may differ from those used in practical photography (previous standards, such as ANSI PH2.5-1979, also specified chemistry and development technique). Consequently, the Zone System practitioner often must determine the speed for a particular combination of film, developer, and enlarger type; the speed determination is commonly based on Zone I. Although the method for determining speed for the Zone System is conceptually similar to the ISO method for determining speed, the Zone System speed is an ''effective speed''<ref> The effective speed determined for a given combination of film and developer is sometimes described as an "Exposure Index" (EI), but an "EI" often represents a fairly arbitrary choice rather than the systematic speed determination done for use with the Zone System. </ref> rather than an ISO speed.
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