Archived posting to the Leica Users Group, 1997/11/17
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]Regarding using cold temperatures and film sensitivity: The issue is reciprocity failure on long exposures. Film isn't used extensively anymore by astro professionals, now it is all CCD. Film is still used by amateurs, and in that case "hypering" which consists of bathing in forming gas causes the film to have much less reciprocity failure. Previous to hypering, the experimenters were getting into "cold cameras" where the film was cooled in order to improve the reciprocity characteristics. I can pull some of the old literature I have on this, if anyone is interested. My understanding of the mechanism of reciprocity failure is that it takes a certain number of photons to "turn on" or activate each silver halide crystal. Once activated the crystal has a rather long life allowing days, weeks, or even longer before development. Before activation, however, the crystal will lose energy, so it takes more photons to turn it on. Sort of like climbing a stair, getting tired, and falling down a few steps. Cooling the crystal slows down the photon loss process, and therefore the film doesn't take as long to make an exposure. We are talking 15-30 minutes long, or longer, by the way. Bob "Astro Lawyer"