Reciprocity (photography)

<p>In <a href="/facts/Photography/ezLPT43M">photography</a>, reciprocity is the inverse relationship between the intensity and duration of light that determines the reaction of light-sensitive material. Within a normal <a href="/facts/Exposure_(photography)/Rzr0aGMd">exposure</a> 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 <a href="/facts/Optical_density/Rnv6VMX1">optical density</a> of the developed film) can result from reducing duration and increasing light intensity, and vice versa.
</p><p>The reciprocal relationship is assumed in most <a href="/facts/Sensitometry/mDWEXXSy">sensitometry</a>, for example when measuring a <a href="/facts/Hurter_and_Driffield/TyboooL8">Hurter and Driffield</a> curve (optical density versus logarithm of total exposure) for a photographic emulsion. Total exposure of the film or sensor, the product of focal-plane <a href="/facts/Illuminance/6Dj4pKmu">illuminance</a> times exposure time, is measured in <a href="/facts/Lux/xa3AAqWd">lux</a> <a href="/facts/Second/jtFxnJXx">seconds</a>.
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Reciprocity (photography) open-in-new

Reciprocity (photography)