All the equations are in the spreadsheets, so you may be able to fiddle with it to get a good fit with the Ilford data.
If you like math and are curious how I came up with the equation, I just plotted the time/temp data with the C64 and then manipulated the x and y axis values (ie log, square, square root, etc) until I found a combination that produced the lowest standard deviation for the line (ie, the straightest line). This was not very difficult, as I had to write my own line-plotting program, and distorting the axes was easy. I think if I were to do it again today I would use some software I have been using to fit polynomials to H&D curves. The only problem I have with that is once I have the polynomial equation, I don't have good way to solve it.
Not sure if PE was implying the Manufacturers temp compensation data is good or not. I always used their temp compensation data. I figured it was like ISO testing; that is, I am not likely to have equipment accurate or sensitive enough to dispute the Manufacturer. Is there any value to test and generate a 'personal temp compensation index,' (like doing your 'personal exposure index')? I don't know, but I am always open to ideas. I just figure the thermal properties of mater are similar between my darkroom and the Manufacturer's lab.
I was pointing out that you have one set of constants for the entire equation, but Kodak points out that the wheel must be adjusted in a non-linear fashion and they give a matrix as I described, of different constants for each film and developer. I was wondering how your formula could do that, as it seemed not able to take into account all of the Kodak constants.
From a table by J.M. Eder,
published many times,
taken here from “Developing”,
by CI Jacobson,
13th edition, Focal Press, 1955.
“The increase in speed of develpoment with an increase of 10˚ Centigrade is taken as the Temperature Coefficient of the developer. For example, if we find that a certain developer works twice as quickly (at 25˚ as it did at 15˚), then we say it has a Temperature Coefficient of 2."
While I think this is only useful as a guide, it IS helpful to see the factors in play.
Temperature Coefficients of Various Developers
Metol-Soda 1.5
Metol-HQ 1.7-1.9
HQ-Soda 2.2-2.6
Pyro-Soda 2.2-2.4
Rodinal 2.0
Glycin-Potash 2.5-2.7
Pyrocatechin-Potash 2.8
This should illustrate why a single correction curve for all developers is invalid. FURTHER, films play a factor. In Kodak published curves,
you can see that D-76 and Microdol-X are sometimes, but not always, parallel.
While a single correction factor is a good estimator,
so is an experienced hunch. A notebook usually is much better .
Rodinal, whose times should double from 59 to 77 degrees F, comes close with some films, but depending upon the degree of development (gamma, or CI). Here are three AGFA curves, for 3 well-missed films. See that the the lines curve, and are seldom parallel. Two Rodinal dilutions are shown, as well as Studional/Rodinal Special.
Last edited by df cardwell; 07-16-2008 at 05:05 PM.
I was pointing out that you have one set of constants for the entire equation, but Kodak points out that the wheel must be adjusted in a non-linear fashion and they give a matrix as I described, of different constants for each film and developer. I was wondering how your formula could do that, as it seemed not able to take into account all of the Kodak constants.
PE
Ok, I think I see what you are saying, my wheel may now be outdated. My 1974 Kodak wheel has a table with numbers in the 30s to 40s, but there is nothing that interacts with the wheel function. For example, in the table in front of me, all 4 film/developer combos that have number 38.5 all have a baseline time of 9 min at 20c and about 10.5 min at 18c based on the wheel. There is no notation that one would spin the wheel farther or differently for each of the 4 different combinations. I agree with you 100% that those different film/dev. combos probably won't respond the same, but my wheel says they do, and the equation follows the wheel exactly. The best thing as you are saying would be a separate equations for each combination, but as far as I knew Kodak never provided this until I saw the T-max literature, at which time I deciphered it to come up with a new equation.
DF has shown it well. IC, note that in that table, you are talking about 3 films in 4 different developers and the fact that they may have the same coefficients is merely coincidence. So, in that table, Royal X pan in Polydol behaves like Super XX in HC110 B. In DK-50, these two films are a 34 and a 36 which would be quite far apart. And as the above charts show, behavior is non-linear.
PE
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DF has shown it well. IC, note that in that table, you are talking about 3 films in 4 different developers and the fact that they may have the same coefficients is merely coincidence. So, in that table, Royal X pan in Polydol behaves like Super XX in HC110 B. In DK-50, these two films are a 34 and a 36 which would be quite far apart. And as the above charts show, behavior is non-linear.
PE
34 and 36 are indeed different numbers and the wheel behavior is non-linear, as I pointed out in my equation for the wheel.
My contention is that there is absolutely no alteration of the temperature-compensation function of the dial based on anything in the chart!
The wheel and chart predict that ALL combinations film/developer with a baseline time/temp of 8min at 20c have the SAME time at 18c!! You can find the time at 18c by using the wheel or the spreadsheet; they give the SAME answer.
Please show me any two film/developer combinations in the chart the have the SAME baseline time at 20c and have DIFFERENT times at 18c based on the wheel and chart!
How can you imply the speadsheet 'may not work' when it gives the same numbers as the wheel?
(PE: this is a 'friendly' gentlemen's argument, so 'no flames' OK. I'll 'eat crow' if you show me to be wrong.)
Well, if I understand you properly, then Verichrome Pan and Tri-X Pan are identical in DK-50 at all temperatures, but are quite different in HC-110B at all temperatures. And, as you point out, since the wheel is non-linear the temperature response is the same in DK-50 but quite different in HC-110B. But look at the post above by D. F. Cardwell if you don't believe me. That is some pretty good data! I have similar Kodak plots.
Please show me any two film/developer combinations in the chart the have the SAME baseline time at 20c and have DIFFERENT times at 18c based on the wheel and chart!
Here.
From the 1962 'Negative Making' booklet from EKC,
showing Plus-X sheet film. This IS representative.
In the Darkroom Data Guide, the Development Dial # for D-76 is 34, for DK:50 1+1 is 34, and for Microdol is 40.
The Development Dial follows the D-76 curve: 68˚ is 8', 75˚ is tad under 6'.
For DK:50 1+1, the wheel and curve chart agree at 68˚, 6'. The wheel specs 4 1/2 minutes for 75˚ while the chart suggests 5'.
The wheel and chart call out 10' for Microdol at 68˚. At 75˚, the dataguide wants 7 1/3; the chart, 7 3/4.
All in all, the D-76 curve is useful for most film and developer combinations, but ACCURATE for none other than D-76 ! Other films show more dramatic differences, but this shows how it works.
The last published temperature adjust charts Kodak published, as far as I know, was in 1962; by the 1967 the charts were absent from the film booklets and the calculator wheel was in the dataguides. The assumption, and specification, was the dataguide was a starting point.Kodak ALSO published more specific data in table form. Until the advent of TMAX films, the published data was taken with a grain of salt. By the time XTOL was on the market, the data from EKC was dead accurate. A lot had changed, in particular, the state of the darkroom was closer to Lab quality than before. But it is just possible there was a different attitude in behalf of EKC.
The Dataguide, and Development Wheels are valuable, but they seem to follow the 80/20 rule. Assigning a precision to them which they were never intended to possess would be a mistake.
My observation over the years was that Kodak's old data was to get you CLOSE, and SAFE.
And, compare D-76 with T-max. At one temperature they are matched, but at another they differ by over 1/2 minute. And, this is the most modern information I can supply. It is quite up to date.
Linear Mode
