Pre Exposure

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.

Will work fine. As long as you keep track of what needs post exposure.

Cheers. DJ

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)

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.

 

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Regards,

I hate to put a damper on things, but I don't think you'll get the

same effect.<br>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.<br>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.<p>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?

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

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,

 

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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.

 

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Regards.

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?

 

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Thanks,

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.

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.

 

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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