Originally Posted by
Mark J
I should probably stop using the term 'Callier effect' in further explanations, because I am likely to be mis-using it. I think that Callier effect strictly refers to the loss of overall contrast in going from a contact print to an enlarged print. It is cause by scatter, so I will just use the word scatter.
Just to continue explaining this from the standpoint of different types of neg illumination, the view up from the negative towards the enlarger source is similar in some ways in the case of a condenser head and a diffuser head ; in that, there is a uniform bright patch of light that is incoherent ( in physics terms ). It's just that in a condenser head, this patch of light is some distance away, and small in angular terms , whereas from a diffuser head, the light fills nearly a hemisphere. It's not a case that one is 'already diffused' while the other is not. It's just that they subtend different angles.
On the other side of the system, we can say that there's no argument about how the light from the underside of the emulsion proceeds to the paper - whatever emerges from the neg and is heading towards the entrance pupil of the enlarger lens will be focused and appear on the paper.
So the bit that interests me, and I think can explain the difference we see in prints from stained and non-stained negs, and between condenser and diffuser-printed pictures, is the bit in the middle - what happens to the light in the film base and particularly in the emulsion.
I really need to draw a couple of pictures to do this properly, but that will take some time, so i will put down some words first.
I believe that what distinguishes the above cases, and how the prints look, is the amount of lateral scatter of the light in the emulsion.
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