What Poco is proposing is that if one exposed a negative(s) to a uniform non image bearing light at a variety of exposures and resultant densities that these could be sandwiched with the camera negative at printing and the effect would be the same as pre-flashing a camera negative provided the camera negative shadow exposure was sufficient and the highlights densities were not lumped on the shoulder.
Originally Posted by davet
The reasoning behind this is that the density range would be compressed at the printing stage in exactly the same way that preflashing does at the film exposure stage.
"No doubt I'm being thick here, but I do not. I understand
"classic" preflashing in the sense of filling in the toe,"
Very full! I've mentioned A. Adams use of Zs 1 & 2 and Donald
has mentioned using Zs 3 & 4. Perhaps some are thinking of
"what does the original poster mean by "sandwiching"?
Is this at printing ..."
My question a few posts back. Yes, "sandwiching" when
"What is the reasoning behind it?"
The reasoning behind it is faulty. Dan
When flashing the negative you are adding a quantity of non image forming light to the image light. This will change the tonal relationships (contrast) of the negative more in the low light shadows and hardly at all in the higher light areas. When you sandwich a uniform density piece of film with the negative you are reducing all the light hitting the paper in the same proportion. The negative contrast is not changed. The same effect as just reducing the enlarger light intensity. If you (in a separate exposure) first expose the paper to non image light and then to the image light you will change the tonal relationships on the paper. This will work opposite of negative pre flashing and effect mainly highlights. How does all this sound?
Originally Posted by ChuckP
Well, at first glance this sounds exactly like my reasoning before I applied numerical values to the hypothesis that Poco proposed.
However, when I began to approach this from a factual standpoint my earlier reasoning did not hold up.
What I finally came to was that if one is preflashing at a Zone I or II value then density is being added to film. It is not factually a matter of simply adding below exposure threshold light.
As I have mentioned, I sometimes preflash as high as Zone III or IV to compress extreme brightness situations. In those instances the density addition is substantial. After a pre-flash of that great an amount, I calculate my second exposure based on highlight placement and not shadow values.
In that case, I don't believe that Poco's conjecture would produce the same result. Whereas it may very well produce a similar result with lower pre-flash placements.
"The same effect as just reducing the enlarger light intensity."
This is what it all boils down to. Would it be the same effect or not? I'm not sure.
We know that the recommended best practice in developing extremely old, outdated film is to overdevelop for increased contrast to compensate for elevated base fog. Obviously that base fog does much more than just reduce enlarger light intensity. The question is whether "fog" provided by a sandwich negative would function in the same way.
I'm far from my darkroom or I could have tried it in the time I've spent yammering about this thing.
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I still don't understand. You're just adding an ND filter at FB+F (or more if you preflash higher) of the neg. How is the paper going to know?
There is another thing to consider here...that is the density range of the camera negative. That is ultimately what will determine the contrast.
Originally Posted by davet
Adding uniform ND will not change the DR of the camera negative. Wheras when one preflashes and then determines the second exposure of the negative on highlight placement rather then shadow exposure, the DR is compressed.
you said, "Wheras when one preflashes and then determines the second exposure of the negative on highlight placement rather then shadow exposure, the DR is compressed."
When one applies this technique is there any development considerations to be dealt with other than just a normal development time?
Originally Posted by lee
That depends on the brightness range (contrast) of the actual scene/subject. An example would be a brightness range of twelve stops. If one preflashed at a Zone III exposure. the effect would be to raise a Zone I placement to a Zone III 1/3, a zone II placement to a III 1/2, a Zone III placement to a Zone IV, a Zone IV placement to a IV 1/2.
So we have effectively raised the lower values and then we can place the second exposure Zone XII value on a Zone X placement (keeps from shoving the high value densities onto the shoulder) and give N-1 development. Thus a twelve stop brightness ratio is compressed to a seven stop density range on the camera negative.
The net effect of doing it this way is that the contrast compression is shared by both the shadows and to a far lesser degree on the highlights.
Sometimes there is no need to alter development at all. Other times in extreme conditions one may alter by N-2. I try to keep changes in development at a minimum...I prefer good highlight tonal separation at the expense of shadows...so long as shadow detail is present...and this method does provide for that.
As I mentioned this depends on the actual conditions prevalent at the time of the exposure. I have used this method to expose fourteen stops of brightness range and arriving at a negative that was capable of printing on Grade 2 1/2 paper.
Don't forget that fogged film is really what pre exposure is. But it is caused by an exposure to non image light. All you're doing with the sandwich is cutting the light hitting the paper. Does it matter if you sandwich a piece of film exposed to .3 density or just change the fstop by one stop? If you measure light through the .3 density film all areas will be one stop down. Highlights and shadows. The amount of light lost in the highlights will be much greater then the shadows. But the contrast isn't changed or the scene tonal relationships.