Reciprocity failure and development adjustmentgreenspun.com : LUSENET : Large format photography : One Thread
Here's two not-quite-newbie-anymore questions:
1) Is there a physically relevant reason that development adjustments for reciprocity failures are given for some films and not others? For example, Kodak E-31 suggests a -20% dev time for most B/W films at the +2 stop speed (10seconds), but no dev time adjustment for TMax films (even over 100 seconds for +2 stops). My understanding (and all I've found in the forum archives) is that contrast is generally increased with prolonged exposure, since clearly the darker areas of the negative will be receiving proportionally less light and therefore the contrast range should increase as these areas suffer proportionally greater reciprocity failure. I can't see why this would be different just because a certain amount of reciprocity failure takes longer to come into effect with some films. For that matter, Ilford doesn't mention dev adjustment for, e.g. FP4+. That could be a matter of company preference, but I don't understand the difference within the same Kodak document.
2) To get more detailed than is probably necessary to just take a picture I'm satisfied with: How do I calculate a reciprocity failure dev adjustment on a sheet scheduled for N+ or N- processing?
2a) N+1 with, e.g. Tri-X at 1+ seconds, might be the natural result of the lengthened exposure, suggesting that if N+1 is desired, I should lengthen the exposure according to the tables, but not adjust development.
2b) N-2 (etc.) seems trickier. I'll hypothetically start with Tri-X, which I will rate at E.I. 320, D-76 1:1 for 10 minutes for N dev. I want to reduce the contrast to N-2, so I estimate E.I. 160, D-76 for 7 minutes (-30% dev time). I note that this EI gives a meter rating of 10 seconds for the chosen f-stop. I adjust the exposure to 50 seconds according to Kodak's table and rush the negative into the D-76. How long do I develop for? Choice 1: 5.6 minutes (10 - 30% - 20%), i.e. serially additive. Choice 1': 5 minutes, i.e. just add 30%+20% and then subtract (surely this can't be correct!) Choice 2: Some other figure, because the calculations can't be simply added. Choice 3: You are going to get into serious trouble not making lengthy expensive tests for each possible combination of reciprocity failure and desired contrast, so no figure can be given as an equation based on your normal development time.
Needless to say, I would prefer to hear choice 1! I may even act according to choice 1 if choices 2/3 aren't going to give dramatically different results, but I remain curious about the "correct" formulation. You can also see why my first question (film types) affects this latter question....
Thanks in advance and sorry for the overly long question -- I couldn't figure out a way to ask it more succinctly.
-- Eric Pederson (email@example.com), March 16, 2001
My question was too short! I realize that adjusting dilution is another factor in all of the above, but I was hoping to keep dilution constant, both to understand the principles and because I prefer not to dilute my developer any further given the small amounts I already use in my individual developing tubes (BTZS homemades with XTOL is what I usually use).
-- Eric Pederson (firstname.lastname@example.org), March 16, 2001.
Eric, Reciprocity characteristics depends a lot on emulsion formulation and if you look around, on John Sexton writings for instance, you may find that at long exposures T-Max 100 can be even faster than Tri-x. People in industry seems to be working hard on these things. As about your math, can't say much, as I've never found myself any significant differences on small percentages up or down anything. Experimental errors usually comes on much bigger numbers. I hope somebody else shall give some better information on this. Greetings, Cesar Barreto
-- Cesar Barreto (email@example.com), March 16, 2001.
Beyond the Zone System by Phil Davis discusses reciprocity failure in one of the last chapters. If I recall correctly, he states that some film/developer combinations will exhibit an increase in contrast while others will actually exhibit a decrease! Beyond the Zone System is a book on sensitometry and not "yet another book complicating the Zone system." (This is beginning to sound like my standard answer to everything, but I love this book!)
As for your math on part 2b), Choice 1 will probably be the closest - sequentially reducing the developing time by the adjustments for the individual phenomena, but the reaction time is most likely not linear, so choice 2 is most correct, but choice 1 would be a starting point.
Choice 3 is also right. The answers to most of the questions on this board come down to "do your own testing."
-- John H. Henderson (firstname.lastname@example.org), March 19, 2001.
>> My understanding (and all I've found in the forum archives) is that contrast is generally increased with prolonged exposure, since clearly the darker areas of the negative will be receiving proportionally less light and therefore the contrast range should increase as these areas suffer proportionally greater reciprocity failure. <<
Eric, I'm not exactly sure what you're trying to say here, but it doesn't sound to me like you are really describing reciprocity law failure (I think you may actually be trying to describe the shoulder of "characteristic curves"). With low intensity reciprocity failure (I think you mean this; exposures become very long), the contrast increase is presumably due to some of the very low intensity exposure somehow "leaking away" or becoming not developable. The higher intensity exposure (which makes darker parts of the negative) do not generally "leak away" so easily; the combination is higher contrast.
If someone could make the case that Tmax films are inherently more resistant to the loss of low intensity exposure, it would explain why Tmax contrast does not increase and thus reduction of development is not necessary. I don't know if this is really the correct explanation, but Tmax films (with "tabular grain") are inherently different from "conventional" films and might have this characteristic. At any rate, I have more faith in published Kodak literature than many and would take them at their word until I tested further.
-- Bill C (email@example.com), March 20, 2001.