Back Bunsen-Roscoe-Gesetz German ضریب شوارتزشیلد Persian Resiprookki Finnish Закон взаимозаместимости Russian Reciprocitetsfaktor Swedish Закон взаємозамісності Ukrainian 倒易律 Chinese
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Note that the times are approximate, and vary between different films and ASA numbers, but the table shows in general how the exposure time is adjusted.
In photography, reciprocity is the inverse relationship between the intensity and duration of light that determines the reaction of light-sensitive material. Within a normal exposure range for film stock, for example, the reciprocity law states that the film response will be determined by the total exposure, defined as intensity × time. Therefore, the same response (for example, the optical density of the developed film) can result from reducing duration and increasing light intensity, and vice versa.
The reciprocal relationship is assumed in most sensitometry, for example when measuring a Hurter and Driffield curve (optical density versus logarithm of total exposure) for a photographic emulsion. Total exposure of the film or sensor, the product of focal-plane illuminance times exposure time, is measured in lux seconds.