G curve - average gradient of a characteristic curve, describing similar characteristics to gamma, but measuring the slope from a line joining the lower and upper limits of the curve actually used in practice.
The scale is identical to ASA (American Standards Association) where the rating is based on an arithmetical progression, using an average gradient system. Therefore ISO 200 film is twice as fast as ISO 100 film but only half as fast as ISO 400 film.
 See also: Gradient, Focus, Space, Shape, Flash
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