Very nice reference.
I would like to point out that the curvature n the density axis as a function of exposure also leads to contrast reciprocity, a subject seldom mentioned.
We know that reciprocity can cause us to lose speed as a function of light intensity, but it can also cause us to observe moderate to severe changes in contrast over the same range. This is known as contrast reciprocity failure.
In a single emulsion film or paper, this may not be a severe problem, but in a product with emulsion blends and in which the emulsions don't respond equally, you begint to get bumps in the characteristic curve.
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Thanks a lot, folks. I have always said that, though I may have a certain 'mastery', I will never know it all. But I now know a little more than I did. And that is what it is all about. You guys rock!
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Chris, thank you for digging deeper into this subject and thanks to all who contributed. Very instructive thread.
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Well now I'm curious. First, by "underexposures of short duration" I would think more in terms of electronic flash exposures like 1/10,000 sec rather than 1/3 sec or longer (which would be closer to the threshold of long duration exposures -whatever they are). Second, it seems several people have described the results of the reciprocity failure effect (i.e., less effective exposure at certain exposures outside a given range) , but haven't really explained why it happens.
Does it have something to do absorption or loss of photons in the latent image (as Mike Wilde has suggested) or is something else going on? I thought it had something to do with the wavelength of light, surface area of silver halides, and probability of photons being in the right place at the right time. Isn't that why T-grain films and higher-speed emulsions have reduced reciprocity effects compared to other films i.e., a higher probability that larger grains or flatter grains oriented a certain direction will be struck by enough requisite photons in a given amount of time?
What was done to make, for example, Vericolor type-L have a different reciprocity range than other standard negative films?
Well, I'm not the person to give a detailed answer on your question. There are others better qualified.
I do know that several things are going on based on grain size variations within an emulsion. The more polydisperse it is, the more non-uniform the added chemicals are in distribution and therefore the more non-uniform their action on the grains. Most of the chemistry in emulsion making is surface related in one way or another and the finer grains have more surface to volume area and adsorb more chemistry based on how many are present.
Therefore, the effect varies from toe to shoulder and that is a function of grain size.
As for Vericolor-S and L, yes, they used different chemical addenda and emulsions to adjust the ranges of reciprocity for short and long exposures. I'm not sure what they used as they had been discontinued when I worked on consumer negative films in the 80s.
Today, the monodisperse emulsions tend to even out these problems by adsorbing chemicals more evenly in a given emulsion, and modern dopants such as Iridium and Osmium tend to minimize or completely eliminate reciprocity failure in most cases. In fact, it is even very difficult to get a modern emulsion to solarize for much the same reason.
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I have noticed something similar to this effect in very long exposures with my 8x10, where I calculate the reciprocity failure into the exposure, and well lit parts gain more density compared to darker areas which expose normally. In essence, I'm getting expansion without processing for it.
Originally Posted by Photo Engineer
That's just, like, my opinion, man...
This is contrast reciprocity failure in action.
Originally Posted by JBrunner
As a point of reference, the older Kodak literature stressed that development should be lengthened when using electronic flash of very short duration, such as that offered by the original Kodatron units. The flash duration was described as being in the 1/25,000 second range, much shorter than a typical studio flash today. Some of the "thyristor" flashes may do this today, but not a 1200 watt-second power pack.
For what it is worth, reliably measuring a quarter of a millisecond with the instrumentation of the 1940s was not a trivial task, so the duration figures are somewhat open to question unless someone has confirmed them with modern gear. At least one citation that I found from 1941 gives a duration of 1/3000 second, which is improbably low given the design of the unit (2000V oil-filled storage capacitor).
There was even a journal for that topic alone. "The Journal of High Speed Photography".
And, the old oil-filled capacitors used to go dry leading to failure.
According to Photographic Materials and Processes, "When exposures are made at high illuminance, with corresponding short exposure times, the electrons are released so rapidly that the relatively slower-moving silver ions cannot neutralize them fast enough for the centers to grow to the predicted size. A greater number of sub-image centers are spread over the crystal surface, and sometimes into the crystal's interior. There is more competition for the additional silver atoms formed, and thus a smaller number of them grow large enough to become permanent developable latent-image centers."
The above is related to the Gurney-Mott hypothesis of latent image formation. Also, high energy reciprocity failure decreases contrast as opposed to low intensity reciprocity which increases the contrast.