Trying out Stouffer T2115 wedge on darkroom papers

albada said:

I've done much work on that as well, and my results are in the table below.

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Mark, on both of your values you have quite amount of green even at grade 4 - why is that? I would assume at grades 4-5 there shouldn't be any green light at all to achiev those grades?

My printing process is typical where I first find out exposure for important highlights and then on second print I find out the grade. So probably I would need to calibrate my system for this; I want to maintain highlights tones as I adjust contrast. I know this shifts middle gray. But printing with maintaining middle gray feels a bit difficult?

@vedostuu: I am unfortunately out of ideas. I suspect the difference could be in the spectral makeup of your blue light source, but that's hard to quantify. I do have a spare of the RGB LED I use lying around and I'd be happy to mail it to you, in case you would want to try it in your setup.

Mark, thanks for publishing your data.

Looking at the difference between the numbers in the table (which I understand as log2 attenuations) I would suspect that the combination brightness-sensitivity of the paper is much stronger for blue than for green illumination. It could be that the blue light source is much brighter or the paper is much more sensitive to blue than to green or a combination of the two. Mark, do you have a table prepared as well for a bright highlight tone?

@ic-racer: Indeed there is confusion about how to read the ISO(R) out of the scan of the step wedge projections. I am by no means an expert on the topic, so I will quickly lay out my train of thought. Looking at the strip "V Blue" I say that the first step with density is 14. I would also consider 15, but step 14 could actually be quite close to the 0.04 density lower limit of ISO(R). Going upwards through steps 13, 12, 11, 10 I reach step 9, this is last "big difference". I can still make out a difference in density between step 9 and 8. I would be surprised if step 9 is much beyond 90% of DMax, so 14->13->12->11->10->9 would be 5 steps, i.e. ISO(R) 75. I have relative confidence in this because I suspect that step 14 could be darker than 0.04 and step 9 is about right, so in turn I suspect that I'm underestimating ISO(R) by a bit. If I understand you correctly, you would only count the transitions 14->13->12->11->10, i.e. 4 transitions for an ISO(R) of 60. Is this correct? Why would you not count the step 10->9 at least partially?

Below are measurements I made about 2 weeks ago. The data is not as nice as I would like it to be, but it's enough to see general trends. I believe my developer was not fresh anymore, which should explain the low DMax. A couple more words of warning: The data appears very "noisy" in the dark tones and I don't yet understand why. Maybe the weak developer plays a role in this, maybe I have a bug in my timing control. The line for ratio 0.0 appears to have the same group of 5 sample twice starting at 0.3 and 0.9 rel log(E). I suspect that this is mismanipulation of the exposure control program on my part.

Shown is a HD chart for 11 different ratios G/(B+G) where G and B are exposure times in seconds. I measured the curves by first preparing test strips using time control and then measuring the densities. I am using Ilford MGRC Pearl and Ars Imago Eco Paper Developer. The three vertical lines are at 0.04, 50% DMax and 90% DMax.


Also shown is a chart ("spread") that shows the difference in rel log(E) between where I estimate a line crosses 0.04 density and 90% (estimated) DMax on the y axis, the x axis is the ratio G/(B+G) of the illumination. I didn't dare label the y axis "ISO(R)" because the data is so dirty. But with some imagination you can see a trend between increasing green illumination and ISO(R)

The blue sensitive emulsion in Ilford papers is more sensitive than the green sensitive emulsion and the cyan sensitive emulsion - yes, there are three!

Those results look very similar to what i got with my janky old durst colour head.
Way past grade 00 to grade 3.5 maxed out
Chasing grade 5 from enlarger is the road to madness in my experience - blue filters, print developers, carbonate to developer, selenium toning negatives, intensifiers.....

Personally I think its best just to see it as turning the magenta or blue up to increase contrast and if the negative still needs more contrast then develop negatives longer or just copy it and develop it to higher contrast or just use intensifier if you only need a little extra.

Beyond that, trying to calibrate it to the paper .... a time and mind void

Craig75 said:

Beyond that, trying to calibrate it to the paper .... a time and mind void

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You are absolutely right. I try not to make this as my hobby. My hobby is still photographing and darkroom printing. My excuse is that I converted enlarger to LEDs with DIY solution and wanted to see how it performs in terms on grading. In practice I already know it works fine for my taste.

vedostuu said:

You are absolutely right. I try not to make this as my hobby. My hobby is still photographing and darkroom printing. My excuse is that I converted enlarger to LEDs with DIY solution and wanted to see how it performs in terms on grading. In practice I already know it works fine for my taste.

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oh yeah you have to run the test to understand what the enlarger is doing and it can be miles away from what you expect it to be doing from the paper instructions in my experience.

there will always be a little part of you thinking.. but where is the rest of the contrast gone? .. but unless the answer comes in a dream then it just be one of life's mysteries

vedostuu said:

Mark, on both of your values you have quite amount of green even at grade 4 - why is that? I would assume at grades 4-5 there shouldn't be any green light at all to achiev those grades?
My printing process is typical where I first find out exposure for important highlights and then on second print I find out the grade. So probably I would need to calibrate my system for this; I want to maintain highlights tones as I adjust contrast. I know this shifts middle gray. But printing with maintaining middle gray feels a bit difficult?

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As @distributed noted, my numbers are log2 attenuations.
As another poster mentioned, paper is much less sensitive to green than blue, so you still need a substantial amount of green for grade 4. But for grade 5, my tables have green turned off, resulting in maximum contrast.
The color of your blue LED might affect maximum contrast. I am using what Cree calls "royal blue", which is 450 nm. That's a little more blue than most LEDs.
Finally, I discovered that Foma's RC paper has slightly higher contrast at grade 5 (blue-only) than Ilford's RC. So to get maximum contrast, try Foma.

Here is a table that maintains highlights (Zones IX or X) across grades:

Grade	Green	Blue
00	0	OFF
0	0.1	4.35
1	0.25	3.5
2	0.5	2.75
3	0.9	2.15
4	1.65	1.65
5	OFF	1.4

This table is almost identical to my second table above that preserves skin tones.

Mark Overton

@albada: Thanks for providing your highlight data. After adjusting for the different brightnesses of our light sources, your and my curve for attenuation pair/ratio vs. grade/range match quite well. They certainly have the same shape It's always nice to be able to compare your results to something similar.

distributed, very nice curves, thanks for sharing. Even making curves, getting the correct point for ISO(R) is not easy. One reason I caution against counting all the gray bars on the test strips is that everyone's dichroic head and LED head will all look like crap because they can't get to Grade 5 if you count all the steps

ic-racer said:

distributed, very nice curves, thanks for sharing. Even making curves, getting the correct point for ISO(R) is not easy. One reason I caution against counting all the gray bars on the test strips is that everyone's dichroic head and LED head will all look like crap because they can't get to Grade 5 if you count all the steps

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I don't understand why such heads can't reach grade 5.
I first printed strips of all grades using my tungsten-condenser head. Then I made many strips with different LED settings until they matched those condenser-strips. For grade 5, pure blue (at 450 nm) matched perfectly.
Mark Overton

albada said:

I don't understand why such heads can't reach grade 5.
I first printed strips of all grades using my tungsten-condenser head. Then I made many strips with different LED settings until they matched those condenser-strips. For grade 5, pure blue (at 450 nm) matched perfectly.
Mark Overton

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Yes! exactly. The problem with testing maximum contrast with a 21 step wedge is that the steeper the curve (more contrast) the harder to get a 'correct' ISO(R) value and the more likely to get an error in measurement when the head is indeed functioning fine.

It is just 'math.' The ISO(R) difference between counting 2 or 3 bands is huge, compared to the other end of the scale where the difference between 15 or 16 bands is negligible.

ic-racer said:

It is just 'math.' The ISO(R) difference between counting 2 or 3 bands is huge, compared to the other end of the scale where the difference between 15 or 16 bands is negligible.

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Never thought of this! Grade 4 and 5 difference is 0.7 steps. So to differentiate 4/5 I think one needs atleast 0.35 step resolution. Is 41 step wedge even enough for accurate measurement?

Yes, more steps will make a smoother curve to graph.

There are a lot of potential other errors in testing paper contrast. Reflections densitometers can be constructed differently and can give vastly different results based on paper surface. So one's own tests might only apply to one's own darkroom.

distributed said:

As bernard_L suggests, if you manage to get your hands on a wedge with .10 density increments, you might get a better view of the steep curve resulting from blue exposure.

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You can get .075 steps with a .15 wedge by exposing the paper twice with it, the second time changing the exposure (via light-level or time) by .25 stops.
For the second exposure, I slid the wedge down, putting it under the first exposure's area on the paper, and also slid it sideways by half a step for ease of gauging similar gray levels.
This technique is slower, but is good for critical work.
Mark Overton

albada said:

You can get .075 steps with a .15 wedge by exposing the paper twice with it, the second time changing the exposure (via light-level or time) by .25 stops.
For the second exposure, I slid the wedge down, putting it under the first exposure's area on the paper, and also slid it sideways by half a step for ease of gauging similar gray levels.
This technique is slower, but is good for critical work.
Mark Overton

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I'm curious to try this out but I don't quite follow how it would work, do the 2 exposures overlap?

gijsbert said:

I'm curious to try this out but I don't quite follow how it would work, do the 2 exposures overlap?

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No overlapping. Here's the procedure in more detail:
1. Expose the upper half of the test-strip using the wedge, while covering the lower half of the strip.
2. Cover the upper half of the strip (that you just exposed), put the Stouffer on the lower half, and expose again, but change the exposure by 1/4 stop.
Now you have two test strips on one strip, with 1/4 stop exposure-difference. That gives you the steps in between the steps on the Stouffer.
Mark Overton

albada said:

No overlapping. Here's the procedure in more detail:
1. Expose the upper half of the test-strip using the wedge, while covering the lower half of the strip.
2. Cover the upper half of the strip (that you just exposed), put the Stouffer on the lower half, and expose again, but change the exposure by 1/4 stop.
Now you have two test strips on one strip, with 1/4 stop exposure-difference. That gives you the steps in between the steps on the Stouffer.
Mark Overton

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Got it, thanks!