The Stouffer step wedge I received today.

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Nicholas Lindan

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Darkroom Automation’s meter ... introduces flare.
Err, no. The DA meter's measurements of density include flare. The meter sees the same flare the paper sees. If you are doing by the book sensitometry this may be a problem unless you are measureing a negative of uniform density. If, however, you are doing work of a more practical nature then including enlarger flare can tell you more about the effect of changes to film exposure and development.

Pays your money, takes your choice.
 

Nicholas Lindan

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... the relationship between enlarger height and print density ...
There is a sticky interminable thread on this subject https://www.photrio.com/forum/threads/factor-for-enlarger-head-height-adjustment.44339/

Darkroom Automation supplies a ruler that does all the calculation for you http://www.darkroomautomation.com/support/stopsruler.pdf Errata: the instructions say "the front of the lens" - cross that out and write in "the location of the lens diaphragm" (a good guess is accurate enough). If you don't have an f-Stop timer (a problem easily remedied, URL in the sig.) then a chart of stops<->time for use with the ruler is available at http://www.darkroomautomation.com/support/stopstableforruler.pdf

And an enlarging meter provides height compensation information. If you don't mind doing compensation with the lens diaphragm then the Ilford EM-10 is available for $10-$20 on ebay - don't pay more! Or see the URL in the sig..
 

Bill Burk

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Err, no. The DA meter's measurements of density include flare. The meter sees the same flare the paper sees. If you are doing by the book sensitometry this may be a problem unless you are measureing a negative of uniform density. If, however, you are doing work of a more practical nature then including enlarger flare can tell you more about the effect of changes to film exposure and development.

Pays your money, takes your choice.

Right. The enlarger introduces flare, the Darkroom Automation meter reading includes the flare that is there. Unless you put the step wedge up against the sensor.

These folks are trying to validate a calibrated step wedge. If they include flare they will draw funny conclusions.

Your meter is great, I’ve been meaning to get one for a long time. At the time of printing it helps to take readings of the actual easel image.
 

Bill Burk

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I recommend accepting Stouffer calibration readings as correct.

Here you can see Chuck_P’s readings with Dkoenig’s Dektronics Printalyzer Densitometer next to Stouffer calibrated readings. The deviation is small through step 11, then grows. This happened with my electronic densitometer as well, it’s simply a case where the densitometer could be better calibrated. I know my Macbeth needs to be calibrated.

I measured a T2115C using a primary visual densitometer. Here you will see every step has some error. That’s just the nature of trying to judge when a spot matches the background. But unlike the electronic devices, the errors do not grow. This is my evidence that Stouffer is right.

17A29BFB-6BDC-4EE6-ABE3-A0501113E6E8.jpeg
 

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albada

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First, I calibrate a reference unit to the best of my ability. This begins with something I call "slope calibration", which is a process that uses a calibrated Stouffer T2120C (21-step wedge) to correct for any linearity issues with the sensor. The way the data is processed, minor step-to-step variations shouldn't cause a problem here. The result is basically coefficients for a correction polynomial.
After this, I perform normal calibration on the unit. That involves measuring an open aperture, and then measuring dead-center on patch 4 of a calibrated Stouffer T5100C (5-step wedge).

Thus, your calibration is based on calibrated Stouffer wedges.
I measured my calibrated Stouffer wedge with the PD densitometer, and it differed significantly from Stouffer's numbers.
One would expect a device calibrated with and then measuring calibrated Stouffer materials to agree with Stouffer's numbers. But it doesn't.
All I can think of is Stouffer's measurements are inconsistent.

This disparity made me measure my wedge in other ways. My goal was to avoid relying on any calibration anywhere. All measurements had the wedge placed on the DA meter's sensor, with a mask over it to prevent light from adding flare or piping. My wedge is T3110C -- 31 steps of 0.1 each, calibrated (which merely means they measured it for me). All measurements used all the LEDs -- red/green/blue. These are good Cree LEDs with negligible spill into the IR or UV, so the light is in the visible spectrum. On the light table, the PD reference wedge and my T3110C wedge both appeared neutral, neither having a color-cast when viewed side-to-side. So I doubt spectral behavior would explain the differences.

1. My first measurements were with the DA meter using the enlarger as the light source. Obviously, I relied on the cal in the meter.

2. To avoid relying on the meter's cal, I changed PWM for each step until the meter matched the initial reading. Step density = log(stepPwm/firstPwm). I called this method "enlarger match" because the meter was used solely as a level-matcher, thus not relying on its calibration.

3. Today's measurements kept PWM constant in case the meter varied based on PWM. Instead, I changed head-height, using the meter as a level-matcher. Density = 2*log(stepHeight/firstHeight), relying on the inverse-square law (I wasn't changing focus, so bellows-factor does not apply).

All three sets of measurements closely agree, as you see in posting #22. I can't imagine how I could have goofed all three.

Mark
 

albada

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I recommend accepting Stouffer calibration readings as correct.

They are correct in your case.
That's the problem with inconsistency -- some (perhaps most) results will be correct.

BTW, what visual densitometer are you using? I didn't know such a thing existed.

Mark
 

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All three sets of measurements closely agree, as you see in posting #22. I can't imagine how I could have goofed all three.

If the optical geometry is "wrong," by which I mean not conforming to ISO 5-2, then you can get different (yet consistent) results. (This would pertain mainly to silver images; C-41 dye images, for example, would have a Callier Q factor close to 1.)

Could Stouffer be wrong? Certainly it could be possible, but it would be a rookie mistake to fail to calibrate a densitometer. It's really hard for me to believe that an outfit selling "calibrated" wedges would be that casual.

Fwiw I'm from a large lab outfit that used to run 40 to 50 control strips per day. Standard procedure, when reading periodic batches, was 1) zero and calibrate the densitometer, 2) read and plot the strips, and 3) recheck the densitometer, both zero and the calibration patch. This was not an unusual routine for serious labs. So again, it's hard for me to imagine that a company selling "calibrated" wedges might fail to calibrate.
 
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Chuck_P

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When measuring a transmission step wedge, it is important to always measure from the same spot. One thing Stouffer does not do, is actually tell you which spot on their wedge the calibration values they provide were measured from. If you measure different points along the same patch, especially for the darker patches, you absolutely will see a difference. When I use mine, I always try to aim for measuring from dead center, and only from dead center.

Thank you Derik for your input on this discussion. So my averaging of three measurements across a step denisty is not what I should be doing. It's challenging, for sure, to ensure that the same spot is actually measured on the step, but I will keep that in mind going forward. I wanted to comment on my measurements of the Stouffer steps on my wedge I received. The P-D measurements for the first half of the step wedge were so close to Stouffer as to be inarguable imo. I realize the 2nd half of a step wedge has the darker densities, and, as you indicate, measurements are getting more prone to variation. This totally makes sense to me now that I learned that Stouffer's tolerances increase with increasing density. I'm assuming that you saw the P-D measurements that I indicated in my post #13. I ran through it again measuring one center spot in each step and came up with essentially the same readings with a .01 variation on several steps with others matching the averaged measurement out of three . I think I've figured out, using the cross hairs, on the P-D how to progress uniformly from one step to the next.

Those P-D measurements of the Stouffer wedge are all well within Stouffer's stated tolerances for each step..........So, i'll ask, it should not be a concern that the P-D measurements are not closer to the Stouffer measurements with steps 12-21.
 
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Here you can see Chuck_P’s readings with Dkoenig’s Dektronics Printalyzer Densitometer next to Stouffer calibrated readings.

Sorry Bill, but I don't see my P-D readings there on your hand written data, none of those match the P-D measurements that I posted earlier.....what am I missing?
 

Nicholas Lindan

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From the posted graphs it seems the first-order differences are down to the spectrum of the light source used for measurement, any color filtration and the spectral response of the detector. Measuring a Stouffer tablet with the various methods shows a fan shaped set of curves. These curves can all be brought to convergence by multiplying the density numbers by a wavelength correction factor - a polite term for Murphy's Universal Finaglng Factor (divide by the number you got and multiply by the number you want). Once established the WCF/MUFF should bring future density measurements into reasonably close agreement.

Ultimate accuracy is neither needed nor wanted. Remember "Good enough is best."
 

Bill Burk

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Sorry Bill, but I don't see my P-D readings there on your hand written data, none of those match the P-D measurements that I posted earlier.....what am I missing?

I posted two sheets. One is mine. The other marked Chuck_P is yours.
 

albada

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From the posted graphs it seems the first-order differences are down to the spectrum of the light source used for measurement, any color filtration and the spectral response of the detector.

You mean spectrum matters? We'll see...

StoufferSpectral.jpg


Nicholas is right! Spectrum matters. I measured again (only every other step) using the Darkroom Automation meter, but using the red, green, and blue LEDs separately (three curves). And the wedge is denser (higher slope) when viewed through red light. BTW, my red LEDs are what LED manufacturers call "photo red": 660 nm. The green and blue curves are identical within measurement error. They are also acceptably close to Stouffer's claimed densities. I guess I'll measure them again using the green/blue ratio for grade 2, and that will work well for the paper-curves stored in my controller. That's the purpose of all this work: Ensure the paper-curves are correct so that prints will hit the correct densities.

That still leaves the mystery of the Printalyzer Densitometer being significantly off when measuring a calibrated Stouffer wedge, considering that the device was calibrated with such a wedge.

Mark
 

Bill Burk

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BTW, what visual densitometer are you using? I didn't know such a thing existed.

Mark
It’s a Marshall Studios densitometer.

Light paths start out equal at one end indicating 0.00 then as you move the carriage towards the film sample the light path through film gets brighter as the path around the back gets longer.

The light around the back is reflected down onto a “field mirror” by a 45-degree pane of glass. You look down through that glass and see the shiny mirror with a hole in it. The hole in the mirror lets you see the film sample, and you turn the knob until both the field and the sample spot match.
 

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albada

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I had to search the Internet and study for 10 minutes, but now I understand how that device works.
Clever!
I don't blame you for refurbishing this one (yes, I saw your pre-refurb photo). It's a rare and interesting device. And still useful.
It's based on the inverse-square law operating at both ends.
I suspect the scale is not pure inverse-square, which assumes a point-source of light, but was modified to account for the size of the bulb. If so, this means you cannot replace the bulb with tiny LEDs -- although they are a better approximation to a point-source, the scale would probably be off. No modern tech is allowed in that thing! 🙂
Thanks for posting this.
 
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I sent the step wedge back yesterday. Stouffer was very apologetic as to the condition of the steps that had the spots on them. I talked to the lady who actually packaged the step wedge for mailing. She indicates with certainty, that the wedge did not look like that when inserted into the envelope and it had to have come from the ink on the calibration sheet packaged with the wedge.

They said this is the first time for an issue like this they have seen and are appreciative that ive brought it to their attention. They've been very positive and reassured me that they will take necessary precautions to fix that from happening again. I'm pleased with how they dealt with me in the process.
 

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I sent the step wedge back yesterday. Stouffer was very apologetic as to the condition of the steps that had the spots on them. I talked to the lady who actually packaged the step wedge for mailing. She indicates with certainty, that the wedge did not look like that when inserted into the envelope and it had to have come from the ink on the calibration sheet packaged with the wedge.

They said this is the first time for an issue like this they have seen and are appreciative that ive brought it to their attention. They've been very positive and reassured me that they will take necessary precautions to fix that from happening again. I'm pleased with how they dealt with me in the process.

That sounds very reasonable. I'm glad you've returned it. Now, you'll have a great step tablet that will work great for years. I've bought several step tablets from them over the years. I look forward to seeing your results.
 

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I sent the step wedge back yesterday. Stouffer was very apologetic as to the condition of the steps that had the spots on them. I talked to the lady who actually packaged the step wedge for mailing. She indicates with certainty, that the wedge did not look like that when inserted into the envelope and it had to have come from the ink on the calibration sheet packaged with the wedge.

They said this is the first time for an issue like this they have seen and are appreciative that ive brought it to their attention. They've been very positive and reassured me that they will take necessary precautions to fix that from happening again. I'm pleased with how they dealt with me in the process.

It always pays to contact the seller first if one has a problem. Almost all of the will be willing to correct the problem.
 

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That still leaves the mystery of the Printalyzer Densitometer being significantly off when measuring a calibrated Stouffer wedge, considering that the device was calibrated with such a wedge.

Mark

There are a few questions I have, which might help explain things:
  • Which Stouffer wedge are you using, and how long ago did you buy it?
  • Does the wedge have any smudges or signs of wear?
  • Which direction is the emulsion facing when you measure it?
  • Have you tried measuring the whole wedge? The numbers you've shown so far only seem to go about 2/3rds of the way into what I think the range of these things is. I'm curious as to whether the graph continues to follow the same straight lines with the same slope, or if it starts to curve at some point.
  • Finally, what happens if you calibrate your unit against this Stouffer wedge instead of the calibration strip that came with the unit? (Pick a point around 2.9D, and call that "CAL-HI", when going through the process.)
 
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There are a few questions.....

The question that I have is.......should the PD measurements have matched more closely, not suggesting perfectly, with Stouffer's readings on steps 12-21 of my stepwedge........as it did so nicely on steps 1-11.

Ex: Step 21 Stouffer's reading is 3.01, the PD reading is 2.94. I read the correct side and there were no smudges. Thanks.
 
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dkonigs

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The question that I have is.......should the PD measurements have matched more closely, not suggesting perfectly, with Stouffer's readings on steps 12-21 of my stepwedge........as it did so nicely on steps 1-11.

Ex: Step 21 Stouffer's reading is 3.01, the PD reading is 2.94. I read the correct side and there were no smudges. Thanks.

I went ahead and took the numbers from two of your posts, put them into a spreadsheet, and graphed/compared them. From a linearity perspective, they actually do match. Where they differ, is that the "high" reference material the two measurement instruments were calibrated against was different.
1674762453265.png


Of course even Stouffer's own processes have limits to their consistency. I've measured a lot of T5100 strips at this point, from approximately the same spot on each one, and the densities I measured ranged from 2.88 through 3.06.

Okay, now back to your specific questions...

First, the PD and Stouffer measurements actually didn't match better on steps 1-11 than steps 12-21. The difference between the measurements actually increases linearly across almost the entire range. It just doesn't become large enough to notice until step 12.

Second, when you get your replacement step wedge from Stouffer, I recommend doing two things:
1. Try this again, and see if you still see the same difference
2. If you do see the same difference, then try calibrating your PD against step 20 on your new Stouffer wedge instead of the strip it came with. After doing that, it'll most likely line up a lot better.
 
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1. Try this again, and see if you still see the same difference
2. If you do see the same difference, then try calibrating your PD against step 20 on your new Stouffer wedge instead of the strip it came with. After doing that, it'll most likely line up a lot better.

I will do that and post the results.
 

albada

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There are a few questions I have, which might help explain things:
  • Which Stouffer wedge are you using, and how long ago did you buy it?
  • Does the wedge have any smudges or signs of wear?
  • Which direction is the emulsion facing when you measure it?
  • Have you tried measuring the whole wedge? The numbers you've shown so far only seem to go about 2/3rds of the way into what I think the range of these things is. I'm curious as to whether the graph continues to follow the same straight lines with the same slope, or if it starts to curve at some point.
  • Finally, what happens if you calibrate your unit against this Stouffer wedge instead of the calibration strip that came with the unit? (Pick a point around 2.9D, and call that "CAL-HI", when going through the process.)

  • The wedge is part no. T3110C (31 steps of .10). Calibrated.
  • Purchased around 1.5 years ago.
  • No smudges, and only a slight sign of wear here and there.
  • Emulsion was down in all prior measurements. But the graph below was measured with emulsion up.
  • The graph below shows all 31 steps.
  • After that cal (using step 30), it follows Stouffer's numbers very closely (.01 off at D=2; .02 off at D=3). But now it disagrees with the transmission reference by .06 at the CAL HI step.
Measuring with emulsion up reduced the error. The right third of the graph disagrees by .04 and .05. Calibrating to my wedge reduced the error to about nothing, but the densitometer now substantially disagrees with the transmission reference. So which calibrated Stouffer wedge is correct?

Here's a suggestion: In the materials accompanying the densitometer, have something prominently tell the user to measure with emulsion up. The natural action of people is to put the emulsion-side down; emulsion up never would have occurred to me.

Anyway, thanks for investigating this difference. Out of the box, the difference was 0.05 at many densities above 1.9. That seems high, so I still believe that Stouffer's numbers are not tight enough.

Mark

Stouffer31.jpg
 

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“It’s always emulsion to emulsion.” A young woman once told me that. I wish I could remember more but it was in context of enlarging and projecting, how to know which way to place the film.

Good for densitometers too so we’ve found.

You’re right, not everyone knows it. It could be a bullet point in the instructions.
 

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Measuring with emulsion up reduced the error. The right third of the graph disagrees by .04 and .05. Calibrating to my wedge reduced the error to about nothing, but the densitometer now substantially disagrees with the transmission reference. So which calibrated Stouffer wedge is correct?
Without a third or fourth reference to compare against, that's a hard one to answer :smile:
I know the calibrated transmission references I've personally purchased references from Stouffer and Acurad don't agree with each other. However, both my Stouffer T5100C and T2120C do agree with each other.

I think the only way to truly settle this once and for all is to pay NIST $5k for their reference, and simply assume its the gold standard.

And don't get me started on the difficulty of trying to solve these issues for color... (but that's off-topic for this thread)

Here's a suggestion: In the materials accompanying the densitometer, have something prominently tell the user to measure with emulsion up. The natural action of people is to put the emulsion-side down; emulsion up never would have occurred to me.
I actually do. I just didn't underline or circle it, so its easy to miss. Right in the measurement instructions of the quick start guide:
1674798464308.png


In general, the rule is basically that the emulsion should face the sensor. In the majority of densitometers, this is emulsion side up. However, in the X-Rite 800 series (810/811/820), its emulsion side down.
 
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