Illogical
If you do the ISO speed test on a non pre-flashed and a pre-flashed film they won't be the same.
Preflashing is no different to any of the other sensitisation techniques.
Whilst it has been common to illustrate this concept of pre-flashing and flare as changing the shape of the curve, with the flashed toe starting parallel & above the un-flashed toe, the reality is that the curve remains constant. The silver responds appropriately to a given amount of exposure.
The placement of ALL the subject matter simply moves right along the curve with more exposure.
Illogical
If you do the ISO speed test on a non pre-flashed and a pre-flashed film they won't be the same.
Preflashing is no different to any of the other sensitisation techniques.
Mark, there is a threshold, i.e. a minimum number of photons required) to make a silver halide grain developable, and that number is higher than one. You can therefore not interpret preflash+exposure as linear superposition of exposures.
At very low exposure levels, we have three stages:
- One silver atom is created, but neither stable nor developable. Unless another silver atom is created within a very short time frame, it will likely turn into silver ion. This is the effect which causes reciprocity failure.
- Two silver atoms are created and form a cluster, which is stable but not developable. In this case you have time to convert such clusters into larger clusters which are developable.
- Three or more silver atoms form a cluster, which is both stable and developable. Once you have reached this stage, preflashes and postflashes will add density but not sensitivity.
A preflash converting stage 1 clusters into stage 2 or 3 clusters will be tricky, as it would have to be applied within seconds or less before or after the actual exposure, but obviously the additional effective sensitivity could make it worth the effort. A preflash which converts stage 2 clusters into stage 3 clusters will definitely add ISO speed, albeit less than a stage 1 ---> stage 3 preflash, and such a preflash should be quite easy to dial in.
You can argue that none of these tricks alter the light sensitivity of film, and you are right. These trick do change the sensitivity to image forming light, though.
As exposure reaches a point where it gets stable we are probably getting close to the bottom of the toe.
Exposure doesn't change the curve. The shape of the curve is fixed by the film and the development, the placement on the curve, right to left, is controlled by the total (stable) exposure received.
Additional flare from a SC lens is concurrent with the main exposure and could be in any or each of the three cases.
I rely on met forecast whether I take SC or MC lenses.
You can post flash as well. I believe Vestal used to hang his film up in the darkroom for a while before he developed it. I don't remember where I read that but it was probably in one of his books. He had a formula for it and I believe he used a green safelight. I could totally be wrong though so check for yourself.
The film doesn't care whether it is pre or post flashed, it only cares how much light hits it. (Actually, the film doesn't care, we do, but you get the gist...)
What the pre-exposure does is alter the shape of the film's initial ("native") characteristic curve, raising and flattening it.
Stephen, not sure how intermittency would be accounted for. But since the pre-exposure is just an additional (known) flare factor, in the tone reproduction transitions I'd either superimpose it onto the standard sensitometrically exposed film curve first, before the subject flare factor, or superimpose both flare factors onto the standard curve in one step.
What you describe is called latensification.
Adding additional image forming exposure moves everything up the curve. Adding non image forming exposure (pre exposure) changes the shape of the curve for the subsequent image exposure, lifting and flattening the low exposure densities. Flare does the same thing as pre-exposure, but does it simultaneously with the image exposure.
Think about it in the same way as flashing printing paper. When you do that, you get a slightly faster, lower contrast paper.
This is one of the (many) reasons why the negatives we end up with are not what we think we're ending up with. When you do a ZS-style EI/development test, you're essentially coming up with a flare-free characteristic curve. However under actual shooting conditions, flare is a wild card. Every frame on the roll (or every sheet, whatever) puts the image on a different characteristic curve, depending on how much flare is present in each scene.
Another way to think about is: The "native" curve + non image forming exposure give you a revised curve, onto which the image forming exposure values fall.
I guess I'm thinking that the "movement" is to the right, and always along the curve, rather than "up".
No flashing increases the dynamic range of film there is no detectable difference in mid tones to highlights.
You are modelling the process like it was a digital sensor.
1-Nope, not thinking digital.
2-Actually believe the technique works in a practical sense.
How does adding exposure change the shape of film curve? I don't believe it does.
A given amount of exposure, on a given film, at a given point, in a given development scheme, should result in a given density,
My thought is that the subject matter in question just slides right on the curve. That extra bit of shadow detail we are trying to get at is simply forced right far enough to reach the toe where it can start increasing in contrast between itself and its neighboring tones.
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