Reciprocity Effect

Assuming that the reciprocity law states (to the effect) that high

light intensity plus short exposure time is equal to low light

intensity plus long exposure time and reciprocity failure occurs at

extremely short times or extremely long times, what can be said for

the extremes of light intensity? In other words, reciprocity failure

pertains to exposure times. But, does the same apply for exposure

intensities? Is there such a thing as an intensity "threshold",

below which, no exposure can be recorded regardless of the exposure

time?

Yes. Long exposure reciprocity failure should really be called low intensity reciprocity failure.<br>At low light levels, exposing film is like trying to fill a leaky bucket. Photons of light knock electrons from the silver-halide crystal lattice, to form potential development sites, but those electrons then simply wander around the crystal randomly until they find the same, or another, 'hole' in the lattice, where they then recombine to destroy the development site again.<br>During a long and low intensity exposure; development sites are being created and destroyed continually.<br>There comes a limit where this recombination is taking place almost as rapidly as electrons are being displaced, and further exposure time is almost useless. That's why low intensity reciprocity correction has a near exponential curve.<p>The recombination time is also affected by temperature; so reciprocity failure increases with temperature, and can be reduced by cooling the camera and film.<p>Exactly the same effect causes the phenomenon of latent image regression, where the latent image will gradually 'fade' from an exposed film after exposure and before development.<br>It follows that if you want to get the absolute maximum speed from any film, then you should develop it as quickly as possible after exposure.<br>Film that's stored at a high temperature after exposure, or left for months before it's developed, will show the same characteristics of poor shadow detail and increased contrast as film that's been given a low intensity, long exposure

Pete has some cool answers!....<BR><BR>In astronomy we would pre flash the film sheets or roll films to get the preflash equal to the fog level; say 0.3 density on the negatives......Then any star exposures would add to this preexisting exposure/fog on the negative..Kelly

I wrote rather detailed response on this topic quite a while ago, but I agree with Pete, the way people usually remember reciprocity failure by exposure time is very misleading about this phenomenon. Long or short exposure duration itself has little consequence as to the departure from ideal reciprocity law -- besides film may saturate or doesn't record anything at all, but it is what is predicted from the normal sensitometric curve and ideal reciprocity law. It is unnaturally weak or strong light intensity that causes the problem. However, recent films (T-MAX 100 and Fuji Acros as well as some color films that also use T-grain technology) employ technology to improve light absorption efficiency of photosensitive material, so this problem has become rather easily avoidable.

 

One more thing. "Reciprocity law" is a good thing and usually what you want. The departure or failure or deviation is usually the phenomenon that requires attention. Using the former term to mean the latter is confusing. It is not that I'm fighting against standard terminology -- those who know it use the term correctly.