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1. Originally Posted by cliveh
I have never heard of post flashing. Can someone tell me when this technique should be used and the advantages it gives?
Well, I can vouch for a definite disadvantage of post flashing, especially at 4pm on a friday afternoon, it can lead to getting arrested...

2. Originally Posted by Michael R 1974
Well the point of controlled tests by contacting calibrated wedges is to eliminate the variables, which enables one to answer questions like this.

Clive, in your analogy see the bricks as units of light. In your "pre-flash" example you are adding 10 units of light to all the tones in your final print before the image exposure. In your "post flash" example you are adding 10 units of light to all tones in your final print after the image exposure. The variance of height is due to the image exposure.
I understand this, but surely the energy response (if I may call it that) would be uniform from threshold onwards, but after exposure to the variance of densities within the negative, a subsequent exposure would not have such a uniform effect (related to reciprocity/intermittancy effect). If I start a car and start driving, I can accelerate faster when reaching higher speeds than when starting off.

3. It should have a uniform effect, at least in theory.

4. Originally Posted by Michael R 1974
It should have a uniform effect, at least in theory.
With minor deviations for intermittency effects and latent image keeping properties, possibly reciprocity law failure depending on intensity/duration of flash. All things that would need to be studied to determine their impact. But I think (I would estimate or predict) these effects are minor.

Consider "preflashing" to overcome "inertia", and "postflashing" the overall gray that you decide you want, whether to establish mood or serve some other pictorial purpose. "Preflashing" is invisible and "postflashing" is gray over the entire image.

In your example, cliveh, the story would be: Consider one brick "preflashing", and 10 bricks the "postflashing". The other bricks are the image.

I believe the result (with the exception of certain effects which we might find are not important to us) is the same whether:

You set out one house brick next to another, next to another, add two to one stack, ten to the next and 30 on the next... Then add 10 more bricks to each stack.

Compared to

Make three stacks of 11 house bricks then add two to one stack, ten on the next and 30 on the next.

Though the result is (or may be) the same, I would suggest teaching a working method that keeps pre- and post- flash exposures separated so you can explain their purposes and know how to adjust them independently.

5. I'm not sure why intermittency and reciprocity would be factors. Consider a highlight tone that doesn't come in on a straight print. You pre-flash to threshold (no tone) and the print exposure adds just enough exposure to bring in light highlight tones. To get the same result with post-flashing, you'd give just enough print exposure so the highlight is brought to threshold (no tone), and the post-flash adds just enough exposure to bring in light highlight tones.

6. Reciprocity law failure would be significant and easily studied, depends if the flashing light source is dim for long time (unsafe safelight) or bright for a short time (electronic flash tube). Intermittency if a pulsed-xenon flashing light source is used. Latent image keeping issues if you decide to preflash a whole box of paper and then store it for a month before using. All these things can be figured out separately.

7. Agree with you there. But does anyone (intentionally) use an unsafe safelight, flash tube, or pulsed xenon to flash paper?

Re Latent image keeping, I agree. Although it would have to be tested and would vary by paper type (or even batch), in general I wouldn't advise storing flashed paper.

In any case I've tried to eliminate all these variables from my tests.

8. Ha! Maybe nobody intentionally uses an unsafe safelight or pulsed xenon to flash, but I've got a big envelope of paper that I flashed in June 2012 and am still using occasionally. I wonder how long it will be before latent image keeping will come into play. I'd think it might be noticeable, because the speed difference from flashing for paper negatives is fairly dramatic and the exposure is fairly critical. Even though I have no densiometer or other objective measure, it would have to be a relatively small effect or I'd notice problems.

I've also kept paper negatives for several months before developing with no obvious visible problems.

But we are talking about different things. I'm guessing your tests could easily reveal differences that might not be noticeable to my eye in a paper negative. Differences that could matter if you were making a highly controlled fine print. I'm not suggesting people keep batches of pre-flashed paper for printing either.

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