Quote Originally Posted by StoneNYC View Post
...Second is, the reason for the 20 minutes = 4 hours is about reciprocity failure, so most films hit failure at 30 seconds, Fuji Neopan Acros 100 has a reported 2 minute failure threshold. So basically after 2 minutes (or 30 seconds for most other films) the normal exposure times stop being accurate and have to be extended exponentially. The film basically stops reacting to the light and needs to be exposed for a much longer time. This also means steadier tripod and camera that can be out on time function (bulb would be ok but you would be standing there holding the cable for an hour...). Does this make sense?...
Almost all films hit problems at 1 seconds...ACROS a little later. In low light/long exposures there is so little light hitting the shadow areas of the film, that there is not enough energy to cause the same rate of change in the silver salts as there are in the mid-tones and high lights. In a normal exposure, there is hundreds the times of light hitting a highlight area than a shadow area...and the shadows and highlights react to the light in a normal proportional manner. In a low light situation, the silver salts of the shadows lag in their reaction to the low light, while the midtones and highlights have enough exposure that they still are exposing in the normal manner. Thus one needs to give more exposure for the shadows, and reduce development to hold back the highlights.