Thanks, Jeff. This is a new technique to an old photographer and one I definately intend to make use of. I am going back to photograph a neat scene on the river near my home. The foliage and tree trunks are heavily shaded, but there is a bright streak of sky through the middle. I had planned to pre-expose some negs to try again, and I will also try some post exposure shots. It sounds like a neat solution to a problem we all have.
Regards,
Thank you all for your most helpful information. As you all know there are a million ways to mess up when shooting large format. It will be nice to leave a step for later on in more controlled conditions.
I'm not an expert on pre-exposure, but instead of fooling with plexiglass, etc., why not just use any uniform thing handy, up close and out of focus, and place it in Zone I or II. Like a small gray card, or palm of the hand, etc?
I have used in pre exposure a yellow-brownish card of about 8x10 size. The colour of the card will decrease the blue cast in shadows. It works, but be careful and make some preliminar tests first.
The whole idea with PRE-exposure is to bring the film up to a more sensitive level so a low light level will trigger the emulsion into action. If you post-expose, the film will still only have the typical shadow exposure, except you will fog the entire film. Pre-exposure is supposed to be presensitizing, not merely fogging. The helpful part is making the low levels more capapble of seeing low light when the actual exposure is made. E.L.
In response to E.L., it does not matter in sensitometric terms whether the added exposure (regardless of whether you call it fogging or pre-exposure) occurs before or after the main exposure. The basic idea is that below a certain threshold exposure, no amount of development will overcome the inertia of the film. It is the total quantum of exposure which counts. The basic idea with a preexposure is that it is additional uniform exposure which thus adds substantially to the shadows but not to the highlights where it is extremely small in comparison to the exposure the highlights get in the main exposure. In other words, the idea IS a small, low level fogging exposure which basically changes the shape of the toe of the characteristic curve making it longer and flatter i.e., giving you a longer scale in the shadows with lower local contrast. Exposure after the main exposure works in exactly the same way i.e., boosting what little exposure the shadows got in the main exposure appreciably above the inertia point but adding little in the highlights. Other techniques exist such as hypersensitization which involve treatment of the film beforehand (with peroxide, if memory serves me right), which work the way you describe. Personally, I believe the point is moot. I have used both pre and post exposure (in picture making, not in controlled experiments) and have not noticed any appreciable difference. If any difference exists, I think its a wash in the normal random error that accompanies the whole process. Just my opinion but I haven't seen any evidence to the contrary. Yet! Cheers. DJ
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