Question about H+D Curves

logan2z · Dec 1, 2020

I thought I understood how to interpret an H+D curve for photographic film, but I was looking at the curves for Kodak Tri-X 400 and Kodak TMax 3200 and I'm a bit confused about what I'm seeing.

Here are the curves from Kodak's data sheets for the two films:

Tri-X 400

TMax 3200

I must be misinterpreting these because they seem to imply that Tri-X 400 is a faster film than TMax 3200. Could someone please help me understand why these graphs indicate that TMax is the faster film?

voceumana · Dec 1, 2020

ISO film speeds are based on use of a standardized developer and agitation method, because a change in developer can change the speed test results--that's a problem for something that is supposed to be a "standard".

Trying to compare the H&D curves for 2 different films when 2 different developers with multiple development times as a method for accurate comparison of film speeds is not really practical. In addition, Tmax P3200 recommended developing times are for "push processing", thus the "P" in its name rather than for normal processing. Thus, the 3200 doesn't come with an ISO speed, but an EI, "Exposure Index".

Also, note that the shape of the toes for the 2 different films are quite a bit different.

But, you will notice that the solid black development line indicates a higher density at the -1.0 lux-second exposure for the TMax film compared to the Tri-X, indicating a higher film speed. So the curves are consistent with a higher film speed for the TMax film.

koraks · Dec 1, 2020

voceumana said:
the solid black development line indicates a higher density at the -1.0 lux-second exposure for the TMax film compared to the Tri-X

Looking at the vertical axis, I don't think this is the case. TriX seems to give something like 2.2 or 2.3 at 11 minutes at that exposure level whereas the 3200P gives something around 1.5 at 12 minutes.

I think the explanation is that the horizontal axis is a relative exposure and not an absolute one.

bernard_L · Dec 1, 2020

Looks like an error, plain and simple. Like, preparing the graphs was delegated to junior staff, and result was not verified before going to print. Ether the absolute calibration for one (or both) film was corrupted, or what was meant to be a relative scale was labeled as lux-sec because it looks more scientific.
Remember the new times for 40TX in HC-110? EK never bothered to confess the error.

voceumana · Dec 1, 2020

Kodak's spec sheets indicate 6 minutes developing time for Tri-X and 12 minutes for P3200TMax, so the two solid black lines are definitely the ones to compare. With this, I am pointing out that the 2 films require different developing times.

Look vertically above the exposures at -1.0 and -2.0 lux-seconds and in each case you find higher density for the P3200 than with the Tri-X. This indicates higher speed for TMax when developed as recommended.

ic-racer · Dec 1, 2020

As pointed out, the X-axis shown on the graphs is a relative axis not an actual exposure.

The way it would work is that the log of the sensitometer brightness is added or subtracted from the indicated X-axis. Without knowing that value from the sensitometer, you can't compare the speed of the films with the presented graphs.

Yes, it is confusing.

This is actually a good resource:

https://www.kodak.com/content/products-brochures/Film/Basic-Photographic-Sensitometry-Workbook.pdf

Another good resource is the EG&G sensitometer workbook. It explains how high speed films need extra neutral density filtration, otherwise the sensitometer light is too bright. They may have used an EG&G sensitometer for the graphs. The extra neutral density would explain the right-shift of the 3200 graph.

If that were the case (that they made the graphs with an EG&G sensitometer) you can compare the curves by subtracting "1.0" from the 3200 curve.

voceumana · Dec 1, 2020

You can have an exposure where the log of the exposure is equal to 0--that exposure is 1. (The log of 1 is zero.) The graphs' X axes are in log scale.

ic-racer · Dec 1, 2020

voceumana said:
You can have an exposure where the log of the exposure is equal to 0--that exposure is 1. (The log of 1 is zero.) The graphs' X axes are in log scale.

Thank you for the correction.

koraks · Dec 1, 2020

voceumana said:
Kodak's spec sheets indicate 6 minutes developing time for Tri-X and 12 minutes for P3200TMax, so the two solid black lines are definitely the ones to compare

Oh I see, I didn't get from your earlier post that you were looking at 6min for trix and 12 min for p3200. The general pattern that you see is that p3200 gives a rather steep curve, which fits with its "multi speed" purpose - put differently, push processing still gives enough contrast for easy printing.

voceumana · Dec 1, 2020

So, well overdeveloped Tri-X shows higher speed than normally developed TMax P3200--this may well be!

koraks · Dec 1, 2020

We can only conclude that if the x axes have the same reference. Since we don't know this, it's impossible to say anything about speed on the basis of these plots.

Bill Burk · Dec 1, 2020

I think I can explain. Here’s the high speed part of my graph paper in meter candle seconds. I called out the ISO and corresponding log mcs. If P3200 could be an ISO 1000 or ISO 8000, the 0.1 speed point when developed to ASA parameters would cross at -3.1 or -3.0 correspondingly. TMAX 400 has no trouble meeting this point at -2.7 but P3200 is nowhere near meeting that speed point parameter.

P3200 is capable of being used at the higher speeds because it has such a special curve shape. It does not get it’s speed at the speed point, it’s getting the speed by working the way you use it.

So my conclusion is... The graph is correct and Kodak deliberately avoids saying ISO speed. Instead Kodak calls it “nominal speed” which doesn’t have an ISO definition.

Bill Burk · Dec 1, 2020

I’ll double check. I think I have a roll to test. The first roll I forgot about testing and shot all 36 frames

ic-racer · Dec 5, 2020

Bill, I think you will find the p3200 about 1.5 stops faster by ASA triangle than Tri-x.

Bill Burk · Dec 5, 2020

I found my negatives and I couldn’t resist. There’s a sensitometry strip on the edge.

Now I was trying to help someone get low fog with this film so I developed in D-76 straight 11:30 at 68-degrees F with Benzotriazole. I got 0.22 base and 0.28 base + fog. So that worked (low fog).

Benzotriazole can reduce speed. Since we are talking about speed that kind of messes up this curve analysis.

Anyway I got -2.7 mcs at the speed point, with 0.7 contrast index. That’s literally the 400 speed point.

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