Pre Exposuregreenspun.com : LUSENET : Large format photography : One Thread
I will be doing a lot of traveling for the next two months and room and weight will be an issue. I am taking the minimum amount of 4x5 stuff. One of the items I am considering leaving behind is my double walled plexiglass pre exposure device. Should I be able to accomplish the same result by doing the pre exposure when I get home ? Although technically that would make it a post exposure. Any reason why the pre exposure can't be done after the fact? Thanks.
-- Paul Mongillo (email@example.com), September 03, 2000
Will work fine. As long as you keep track of what needs post exposure. Cheers. DJ
-- N Dhananjay (firstname.lastname@example.org), September 03, 2000.
Paul, What a great idea! It's things like this, which never occur to me for some reason, that make this forum so worthwhile! Not only can you wait till you get home to make your "post-exposure", you can expose a few sheets in the field, develop one after post-exposure and adjust the post-exposure for the others a bit to fine tune if necessary, all in the comfort of your own living room! Regards, ;^D)
-- Doremus Scudder (ScudderLandreth@compuserve.com), September 04, 2000.
Paul: I would think you might want to do some tests first to see if you get the effect you are looking for. Pre-exposure helps mask some of the contrast in a scene, but it also helps to bring the exposure level of the shadows past the threshhold level of the film. Post exposure may work fine, I've never tried it, but I would see if it works o.k. before risking a lot of work and negatives. I would certainly test for the shadow areas. I'm not trying to throw a wet blanket on your idea. Hope it works out great for you. Let us know the results.
-- Doug Paramore (email@example.com), September 04, 2000.
I hate to put a damper on things, but I don't think you'll get the same effect.
Latent image regression will most likely knock out the very low levels of exposure that you're trying to capture, before you can give them the post exposure.
You could always carry a few sheets of pre-flashed film with you, but again, you'd have to give more than your usual pre-exposure to accomodate the regression between pre-exposure and exposure proper.
Do you regularly take pictures of fireflies by starlight BTW? Why not just give more exposure, and pull the developer a bit? Or use a less contrasty film?
-- Pete Andrews (firstname.lastname@example.org), September 04, 2000.
I use pre-exposure all of the time. Most contrast situations I find in my scenes require it. I used to think that pre-exposure worked best and post exposure was for other applications. Having run some pretty precise tests I was proved wrong. Post exposure is the same as pre-exposure. And now I can really tailor my post exposure precisely. Don't hesitate to use it. But keep precise notes of each exposure so when you get back it will be easy on you. But it is such a valuable tool, don't hesitate to use it's advantages help those shadows and the contrast range within the scene. James
-- james (email@example.com), September 04, 2000.
This may sound dumb (I haven't tried pre or post exposure yet) but can't you mount a plexiglass piece into a filter ring and have it occupy much less space? Doesn't anyone market this? I can understand the advantages of a dual-walled device to ensure eveness of exposure. What plexiglass can be recommended for this, are there neutrality problems? Last question: what lighttight boxes are on the market for unloading filmholders in the field?
-- Wayne DeWitt (firstname.lastname@example.org), September 04, 2000.
you need a Wallace Expo Disc, from (you guessed it ) Wallace Photo Products. Should do the job. He has a web site. You can also use it to check your light meter, or to expose sheets for Zone I and Zone VIII to throw in with a batch of exposed film occasionnally-to use as a rough Exp/dev control. If you stick one on your enlarging lens you can pre expose (flash) your printing paper, in the field use it for pre exposure of film.
-- Hans Berkhout (email@example.com), September 04, 2000.
James M.: Glad to see someone ran some test on a process I'd never even thought of. It sounds like it could be a really good tool. All of us come back from shoots where we wished we had had a few pre- exposed sheets. Could you post a few more details, such as how much post-exposure you give and how do you judge how much to give?
-- Doug Paramore (firstname.lastname@example.org), September 05, 2000.
Doug, I have used this approach for the last 10 years, ever since John Sexton helped to save an exposure the he said would other wise not turn out. I was photographing in the same location as his Grotto Interior photograph, which is a cave with an opening at the top. The problem was plenty of light at the top of the image and very little light in the shadows. I was using T-Max 400 and gave an exposure of 5 minutes at f/22. John walked in during the exposure and asked the details of the exposure that I was making and he suggested that it would not be enough. When we got back to where the workshop was being held we added a Zone II exposure to the exposed film and that image prints with a full range of detail. Generally, I add a Zone II exposure when ever using this technique. A Zone I exposure would probably also be acceptable but even at Zone II I find it easy to print threw even if it is too much. Give it a try.
-- Jeff White (email@example.com), September 05, 2000.
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.
-- Doug Paramore (firstname.lastname@example.org), September 05, 2000.
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.
-- Paul Mongillo (email@example.com), September 05, 2000.
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?
-- John Sarsgard (firstname.lastname@example.org), September 05, 2000.
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.
-- Jan Eerala (email@example.com), September 05, 2000.
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.
-- E.L. (firstname.lastname@example.org), September 05, 2000.
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
-- N Dhananjay (email@example.com), September 05, 2000.