Film-Pre-exposure/Post-exposure

[lensmagic]

Ansel Adams, in The Negative, writes on the topic of the pre-exposure of film, in which the photographer makes an exposure of a uniform, non-image, illumination placed on a chosen low zone, after which the normal exposure of the subject is given on the same film or frame. What would be the effect, if any, of REVERSING the order, that is, making a normal exposure of the given subject, then later adding the non-image, low zone exposure???

 

[Paul Verizzo]

Ansel Adams, in The Negative, writes on the topic of the pre-exposure of film, in which the photographer makes an exposure of a uniform, non-image, illumination placed on a chosen low zone, after which the normal exposure of the subject is given on the same film or frame. What would be the effect, if any, of REVERSING the order, that is, making a normal exposure of the given subject, then later adding the non-image, low zone exposure???

It's called latensification, IIRC. Haist has a fair amount about it. I have an old magazine format photo book on increasing film speeds and it is a very thorough examination of techniques and developer by developer comparisons. So much information that used to be available no longer is.

Oh, I wandered. You can also do things like use organic vapors from, for instance, acetic acid or friendly stuff like mercury. I think what they do is bring the film up to base fog level so that any light hitting it makes a latent speck right away.

 

[lensmagic]

It's called latensification, IIRC. Haist has a fair amount about it. I have an old magazine format photo book on increasing film speeds and it is a very thorough examination of techniques and developer by developer comparisons. So much information that used to be available no longer is.

Oh, I wandered. You can also do things like use organic vapors from, for instance, acetic acid or friendly stuff like mercury. I think what they do is bring the film up to base fog level so that any light hitting it makes a latent speck right away.

Hi, do you have the title/author for the old magazine format photo book you describe??

 

[Paul Verizzo]

Hi, do you have the title/author for the old magazine format photo book you describe??

I'll have to get it from storage. Of course, I just put it there a week ago! :rolleyes:

I think this is it here: http://www.amazon.com/Increasing-sp...=sr_1_1?ie=UTF8&s=books&qid=1219521402&sr=8-1

 

[Paul Verizzo]

Here's a list of all of the Petersen photo books. A lot of good titles still applicable.

Dead Link Removed

 

[Neal]

Dear lensmagic,

You may find this useful: http://web.archive.org/web/20060613002051/photography.cicada.com/zs/chapter8/

Neal Wydra

 

[J Ollinger]

Short answer is: I don't know.

Longer answer: the idea behind pre-flashing is this (for anyone reading this who doesn't already know): there's a certain amount of exposure that accumulates before you actually get an image. I'll use "units" of light as simple example. You expose film or paper to 10 units of light. You develop and you get Nothing. It's as if you didn't expose it at all. But if you expose the film or paper to 11 units of light, you get an image. Ten units is the threshold you have to pass to get something you can see.

So with pre-flashing, what you're doing is exposing the film (or paper) to that 10 units of light that's just under the threshold, so that any additional amount, no matter how little you add, will register. That's how you get a usable image in the low-density areas, because you've already built up that base that otherwise doesn't register.

It seems to me (in theory anyway) that post-flashing would have the same effect. But I've never tried it and I've never read a study where someone tried one vs. the other. It would be an interesting project to try.

 

[lensmagic]

I'll have to get it from storage. Of course, I just put it there a week ago! :rolleyes:

I think this is it here: http://www.amazon.com/Increasing-sp...=sr_1_1?ie=UTF8&s=books&qid=1219521402&sr=8-1

I have ordered the book. The price is certainly right ($1.20 plus shipping). I will do some testing and report back. Thank you for your suggestion.

 

[Claire Senft]

I would suggest that post exposure and latensification are not at all the same.

 

[gainer]

I would suggest that post exposure and latensification are not at all the same.

Hardy and Perrin in "Principles of Optics" state that the auxiliary exposure can be given before, during, or after the regular exposure. Its effect during the regular exposure can be seen when the technique is used in enlarging, if you have an easel photometer that reads a small area. I made a little illuminator that had an old iris to control diffused light. I attached it beside the lens and operated it by the same switch as the enlarger. I could set the shadow exposure with the enlarger light alone and adjust the auxiliary light for the highlights. The light added will not affect noticeably the shadow illumination because of the logarithmic relationship between illumniation and exposure. The paper doesn't care whether it is image light or fog light, but both should be of the same color temperature to be most controllable. It is of more use on graded paper than on VC.

 

[Paul Verizzo]

I would suggest that post exposure and latensification are not at all the same.

I believe the general terms are latensification for pre-exposure and intensification for post-exposure. But as so many things photographic there is both disagreement and just plain old sloppy communications.

 

[Claire Senft]

I would suggest that you are even more incorrect than when you started.

 

[Paul Verizzo]

I would suggest that you are even more incorrect than when you started.

So please enlighten us.

Why the attitude?

 

[keithwms]

There isn't a simple answer that is also comprehensive; it isn't quite as simple as predicting the effect of preflashing, which we generally take to mean pre-exposure of ~3-stops less than the main exposure (usually done for reining in scene contrast). Post-flash may give results similar to preflash in some cases, but at the extreme end it may also take you into solarization/Sabattier.

The effects of pre- and post-exposure depend very much on the integrated intensity of the flash, relative to the that of the exposure. It also depends at what wavelength the second exposure is done, for example with IR post exposure you can get the Herschel effect.

In summary, what you get when you preflash and then expose normally is not necessarily the same as what you get when you reverse those. One has to reflect on the Gurney-Mott and Mitchell theories to see why that is so; let me suggest beginning with the Wikipedia article on the latent image as a quick primer.

 

[Claire Senft]

I would suggest that for there to be latentsification that there needs to first be a latent image. A latent image I would say is not the same thing as a non-existant image. Therefore, it is something that happens to, in this case the film, after exposure. At least twofold are the means for doing this. One is done by chemistry, for example bathing peroxide treatment in an alkaline atmosphere or prolonged exposure to extremely dim light. I am now going to confine myself to the use of light to strengthen the latent image in subsequent statements. What is here accomplished is to make developable silver that have been exposed to photons insufficient to become developed. This requires at least a 15 minute exposure to very low light. It requires an absence of other light. It should be done in such a manner...one needs to test for this...so as to produce the lowest level of fog with the maximum of increasing these amount of very faint original shadow exposure that would otherwise be part of F+FB. It will not work for film that has received exposure to very low light levels requiring a second or more of original exposure. This will slightly reduce the contrast of the image and probably require about 5% more development. This will provide increase in film speed. This is not pushing. This is not hyper-sensitation. What does this do do for film speed? Well I latensify all of my film. I use both low light and peroxide treatment a matter of course. I use a Minolta Auto meter IVF. This is a incident meter. When I set the film speed to 125 and point the meter's dome at the unobscured noon day summer sun it returns an value extremely close to f16@ 125th of a second. I have shot a great deal of Tmax 100, and Pan F+ and now I am working almost exclusively with Imagelink HQ which I bulk load. I take a shadow reading when using the Tmax 100 with the film speed set to 400 or 200 in the case of Pan F+. This methodology of exposure is based on BTZS as promulcated by Mr Phillip Davis. I am more than comfortable that the readers of this post or more than familar such technique.

The resulting negatives are rich in shadow detail and will manitain all densities that were in the subject...0 thru the highest value.

This is latensification.

Pre-exposure and post-exposure are but both a means of fogging the film to some particular value. This fogging of the film is done to reduce contrast. If I were to pre-expose...or post expose on, say, Zone II than any zone lower zone II no longer exists on the negative.

I seem to recall that someone wrote a rather lengthy APUG article on latensification sometime in the past. Perhaps it can be retrieved.

 

[Claire Senft]

So, I have a question. Why does someone who asks a simple question about pre-exposure vs post-exposure be treated to such a lot of hot air?

I am guessing that it makes no difference. I am certain that one can find out very simply. Under the same lighting conditions expose one frame, or sheet of film prior to the main exposure. Repeat the process by making the fogging exposure after the main exposure instead of before. Develop the fames identically. Print them.

Now, you know much more than the rest of us about it. The time involved was less than what was expended on all the previous posts.

Go for it.

 

[richard ide]

I have been using post or pre exposure to just below base fog level (actually I prefer about .05 density above base fog) for many years. The cumulative result is the same so order does not matter. Hypersensitization is chemical (mercury vapour) or heat treatment of an emulsion to increase emulsion sensitivity (speed increase). This increases sensitivity to very low intensities of radiation and/or regions in the emulsion with weak spectral sensitization, especially toward the long wavelength limit. This is used for astronomical subject matter. This treatment reduces stability so the film must be used and processed soon after treatment. Paraphrased from the SPSE Handbook.

 

[gainer]

When one adds light in an amount equal to what we used to call the inertia of the film, it both reduces contrast and decreases the inertia without sensibly increasing fog. David Vestal wrote about it for Photo Techniques some years ago. I believe it was still "Darkroom and Creative Camera Techniques". His technique used very long exposures to very dim green light from a safelight.

 

[Chuck_P]

What would be the effect, if any, of REVERSING the order, that is, making a normal exposure of the given subject, then later adding the non-image, low zone exposure???

Just trying to think it through after reading up on it---I come up with this. I've never used pre-exposure but I think order does matter, IDK for sure. But.......

Pre-exposure is used to boost the low values in small areas of the final print. Boosting them "without" pre-exposure would require additional exposure to the negative which could then push the high values so that contracted development is needed to control them. Contracted development (especially N - 2) will reduce local contrast in the middle and low zones. Pre-exposure provides an "out" for increasing negative density at zones I / II without blocking the upper zones; thus eliminating the need for contracted development to control the high values.

Pre-exposure has a much greater affect in the lower zones than in the upper zones. Using pre-exposure first and then adding the main exposure, increases density in the upper zones to no real change.

So, by reversing the order and providing main exposure first and THEN adding the additional exposure, it seems to me that you will simply be adding exposure to the high end of the scale when you don't want it there. It is given that the main exposure has taken them into account already-----why add more to them to block them up on the negative? Doing so would require contracted development, which defeats the purpose of using pre-exposure.

Just my 2 cents. I know someone wiser will correct, as they should.

Chuck

 

[richard ide]

The film has absolutely no idea what order the exposures were.

 

[Claire Senft]

Mr Gainer is completely correct except that it was an article written by Ralph Steiner and edited by Mr Vestal. A very fine article it was indeed. This article was the basis for the APUG article to which I referred.

For those of you who are unfamilar with Mr Steiner he was a chemical engineer by education...Dartmouth if my memory serves me correctly. He was a contemporary and friend of Walker Evans. I believe that the relationship between Mr Vestal and Mr Steiner was one of Mr Steiner being, among other things, friend, confidant and mentor to Mr Vestal.

Mr Steiner was a commercial photographer who also worked in cinema and was a very much respected member of the photographic community.

 

[gainer]

Yes, I remember that now. I have had a case of meningo-encephalitis in the meantime. Things trickle ito and out of my memory still.

 

[gainer]

I think the idea for a long, low level post exposure stems from the reciprocity behavior that causes changes in gradation, especially of the films at the time of Steiner's work. Film exposed for long periods at low levels in order to counteract film's refusal to follow the staright and nrrow path also had to be developed differently in order to maintain desired contrast. Howard Bond in his report in Photo Techniques did not find that effect to be significant in the films he tested, for the range of exposure times he tested.

It is a confusing situation. Adding a uniform dose of light is not the same as opening the f-stop more. Opening the stop changes illumination at every point of the scene by the same ratio. Adding uniform illumination changes the shadow illumination more than the highlight in terms of f-stops. It is probably best answered by controlled experiments using modern films to test old methods.

 

[lensmagic]

So, I have a question. Why does someone who asks a simple question about pre-exposure vs post-exposure be treated to such a lot of hot air?

I am guessing that it makes no difference. I am certain that one can find out very simply. Under the same lighting conditions expose one frame, or sheet of film prior to the main exposure. Repeat the process by making the fogging exposure after the main exposure instead of before. Develop the fames identically. Print them.

Now, you know much more than the rest of us about it. The time involved was less than what was expended on all the previous posts.

Go for it.

You are of course correct, and I knew that when I made the initial posting. I just thought to myself, rather than do the work you have just described, maybe the answer is simple and obvious to everyone but me; but now I learn it is not! So yes, it's into the darkroom I go.

 

[richard ide]

If this will help, I can give you a starting point. I have a Kodak beehive safelight with a 7.5 watt lamp and an OC filter and a light dimmer about 15 feet from my film. My exposure time is 3 seconds for TMax 100. I made 5 darkslides out of black plastic with a 1/2" hole in each (different positions) along one side. This gives me 10 possible exposures on the same piece of film by flipping them over. If you do not have a densitometer, pick the exposure which gives you a discernible density above base fog. I can test a film using one sheet. Sometimes I pre-expose several sheets and other times I expose after shooting if I think a sheet needs it.