Film testing with Pyrocat for VC paper

I'm looking to start using Delta 100 with Pyrocat and I though I'd do a simple film test using a step wedge with a densitometer. My end result is printing on variable contrast papers. I use a home built LED head with green and blue LEDs which I have calibrated for different papers. I exposed a sheet of film through a step wedge under an enlarger using an incandescent bulb for 1/2 second and developed it for the recommended time for Pyrocat MC.

With normal developers there's very little difference between the blue and the green readings and I just use the visible channel to make a density plot for contrast measurements. But with the stain there's about a stop difference on the densest patch (1.31 Green vs 1.57 Blue). I'm looking to see if my thinking about this is correct.

If I plot the green and blue curves I'll end up with two different slopes with the blue being higher contrast than the green. My hypothesis is that when printing at normal grade the effective contrast will be about 1/2 way between the two since the ratio of blue and green light is approximately equal. I'm thinking this will result in a lower contrast contribution from the green and a higher contrast contribution from the blue. It would then go that as the grade is reduced print will be even less contrasty than expected since the low contrast green curve will effect the print more, and the opposite will occur with increasing the grade.

Based on that I expect the print to respond more quickly to contrast changes, but to otherwise print the same. Does that match your experience?

Ultimately I'm wondering if I can design a way to test stained negatives for contrast and speed at different development times (for example BTZS testing), but it feels like we have too many variables to solve for.

Not to wax dumb here, but... Shouldn't the blue curve always be steeper than the green? That said, the printing characteristics of a pyro stained negative will be different on VC papers, depending on the pyro used; pyrogallol stain will produce different paper contrast (mostly in the high values) vs pyrocatechol stain. Gordon Hutchings and Steve Sherman have written quite a bit about this.

Alan9940 said:

Not to wax dumb here, but... Shouldn't the blue curve always be steeper than the green?

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As I understand it, the blue and green sensitive emulsion components do not differ with respect to contrast, but instead differ with respect to speed.

L Gebhardt said:

My hypothesis is that when printing at normal grade the effective contrast will be about 1/2 way between the two since the ratio of blue and green light is approximately equal.

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When working on my own led head, I reached the conclusion that "normal" (ca grade 2) contrast is definitely not a 1:1 blue:green ratio and that the green emulsion of VC paper is a few stops slower than the green (which makes sense, of course). Equal amounts of blue and green resulted in roughly grade 4 in my experiments. Hence, I think your best bet is just to plot some curves for the different contrast settings of your head, as there will be some non-linearity in the system here and there.

Alan9940 said:

Not to wax dumb here, but... Shouldn't the blue curve always be steeper than the green?

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With nonstaining developers the negative is effectively neutral so the curves are the same when read with green and blue light. The effect here is that with blue light the paper will see a different brightness range than with green light.

koraks said:

When working on my own led head, I reached the conclusion that "normal" (ca grade 2) contrast is definitely not a 1:1 blue:green ratio and that the green emulsion of VC paper is a few stops slower than the green (which makes sense, of course). Equal amounts of blue and green resulted in roughly grade 4 in my experiments. Hence, I think your best bet is just to plot some curves for the different contrast settings of your head, as there will be some non-linearity in the system here and there.

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I reached the same conclusion regarding the brightness levels, but I was trying to keep it simpler when mentioning equal proportions.

MattKing said:

As I understand it, the blue and green sensitive emulsion components do not differ with respect to contrast, but instead differ with respect to speed.

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I found confirmation of that at http://www.darkroomautomation.com/support/appnotevcworkings.pdf. That does further complicate my thinking about the problem, but with different negative contrasts based on light color it might actually make the VC paper behave differently.

Since the paper "sees" less green density in the high density areas of the negative, the blue emulsion takes precedence and hence contrast will be greater in those areas than in low density areas. I think that would be my spin on it - of course, what it actually looks like...do some printing!

@MattKing, @L Gebhardt, thanks for the clarification...I did not know that. Learn something all the time on these boards!

I always measure pyrocat-hd negatives using the blue channel on my densitometre. And yes you can make curves at various development times using pyrocat. I've been doing it for 20 years. It only takes 5 sheets of film, and a step wedge. After all the curves have been drawn, and development times pulled from them, I test a couple of sheets outdoors, to verify the times are correct as well as the EI. It's quick and easy...