A while back, I wrote a post questioning whether Barry Thornton or Bruce Barnbaum were right on the subject of pre-exposure of film as an aid to controlling contrast. Briefly, Barnbaum didn’t think it worked and Thornton did.
My natural inclination was to side with Mr Barnbaum since he had a cool first name but Barry had done the tests and said it did. You can read what I wrote here, https://onlinedarkroom.blogspot.com/2015/04/barnbaum-v-thornton-who-right.html?m=1
Towards the end of the post I said, “ I know I have some very knowledgeable and experienced readers and was wondering if anyone has an opinion they'd care to share. Is anyone using pre-exposure and have you done any testing that supports the theory? Or have you tried it and found it makes no real difference?”
Almost five years later, reader Ruediger Hartung got in touch with an explanation that makes sense to me. His comment deserved it's own post. Here’s what he had to say:
“I have tested the pre-exposure of black and white negative film with different films and developers. In the end, I find it a simple, practical method to use with 35mm film on location for individual shots when the contrast range is too high. Therefor I simply use my smartphone with a white surface closely attached to the lens and pre-expose at zone I.
"Thornton and Barnbaum are both right. The sensitivity of the film in the shadow area is raised but the density curve also flattens there, indicating less tonal separation.
"However, assuming that 90% of analogue photographers scan their negatives nowadays, the contrast in the shadow areas can simply be raised again afterwards and having some structural information is better than no information in the shadows!
"But also when enlarging you have possibilities to raise the shadow contrasts again, for example by using a lith developer as the first bath in a two-bath process.
"I would also like to mention that especially with high-resolution microfilm / technical film, pre-exposure has decisive advantages. It usually leads to a "normal" density curve first, but there is more to it than that:
"If it were only a matter of the theoretical resolution values of document films at high contrast 1000 : 1, the use of high-resolution document films for image-based photography would not make sense anyway.
Since even the best lenses cannot exploit the resolving power of these films because of the physical limitation due to diffraction.
"Moreover, with this approach, the results of today's high-resolution films would be almost identical to the results that could be achieved with the Kodak Technical Pan, which could still resolve 320 lp/mm at high contrast 1000 : 1.
"This way of looking at things cannot explain the actually exorbitant quality advantage that high-resolution document films have over conventional black-and-white films, because it is incomplete.
In pictorial photography, we are not normally dealing with high contrasts, but with medium to low contrasts. At these contrasts of about 64 : 1 down to 1.6 : 1, conventional film shows such a low resolution at low detail contrast that it is far from being able to reproduce the lens performance!
"As a result, especially in the range of medium and low contrasts, an enormous difference in quality becomes visible in favor of the high-resolution film. This only makes it clear what performance reserves lie dormant in the lens that could not be used until now.
"In numbers: The best conventional black-and-white films resolve at most 60 LP/mm at a subject contrast of 1.6 : 1, with extremely weak detail contrast in this range and therefore poor sharpness. Photographers have become so accustomed to this quality that they consider it normal.
"Document films with suitable developers, on the other hand, resolve a tremendous 250 lp/mm at the same subject contrast of only 1.6 : 1. This is more than even the best conventional B&W films can resolve at high contrast 1000 : 1! Moreover, even at this low contrast, the detail contrast and consequently the sharpness are exorbitantly high."
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