[Sian Barnell]

hello! i was just wondering, can anyone help me, i am trying to find the correct deffinition for 'reciprocity law departure' as im looking at long exposures and lighting effects and this is something i have to consider. please help!!! thanks! sian

[stupot]

Here's the definition from the 'Oxford Companion of the Photograph' of reciprocity law.

"The Reciprocity Law states that photochemical effects depend solely on the light energy absorbed. It underlies the relation between exposure duration and f-number (e.g that 1/30s at f/11 gives the same result as 1/60s at f/8). The law tends to break down at low light levels when the indicated exposure duration exceeds a few seconds. If the law is followed rigidly the result will be underexposure, and possibly poor colour balance. Increasing the exposure duration corrects the underexposure, but often with increased contrast. Film manufacturers sometimes provide tables for exposure compensation and correction filters."

This from Wikipedia on reciprocity Reciprocity (photography - Wikipedia, the free encyclopedia)

Hope that helps Sian

[sejanus]

The thing to remember is that reciprocity failure only applies to silver based photography, as it's a function of the energy required to overcome the inertia of the silver atoms to the action of light. Digital imaging works in an entirely different way, so you're dealing with almost the opposite effect: at long exposures, random energy excitation of the individual receptors gives rise to unexpected firing of the receptor and thus the generation of unwanted bright spots in the image.