Understanding Reciprocity Failure

[charjohncarter]

a somewhat similar question... is reciprocity cumulative? ie: is the film not able to register long exposures of light all at once because of some chemical threshold gets reached? As a metaphor... you can't pour a beer into a glass all in one go because foam builds up, but you can pour the beer into the same glass if you do it in short spurts with no problem. So one, three second exposure would suffer from reciprocity but would a dozen quarter second exposures on the same frame also be subject to reciprocity?

Or are there two reciprocity failures happening in many or maybe most night images? One, of course, is the dark under light areas of the negative. But, two, there is usually an area that is light by some man made kind of lighting; street light, signs, car headlights, etc. If the man made lights or area were metered they may well not be in the times of reciprocity failure. So add failure for your scene and the light areas a completely blown. Maybe not important to some of you but for me I don't like that look so I try to have both areas in the reciprocity 'zone' this of course doesn't completely solve the problem but it does give me a little more of what I personally like. Here is one that I stopped down to F 22 not for DOF but to get both light and dark areas in the reciprocity area.