Densitometry and Pyro developer.

I have a question that has been nagging me for some time. Everywhere I read and in this forum it is stated that you cannot use a B&W densitometer to read a pyro negative because the stain interferes with the reading,Tthe "sees" the stain as "density" and what we really are concerned is silver density. My question is wh y? shouldn't we be concerned with OVERALL density wether it is silver and/or a c onbination of silver and stain, isn't it better to have a reading that reflects how the enlarger light "sees" the negative, with stain and all? For example if I take a reading in the blue channel and I get zone VI, but when I print it it is about VII or more (which coincides with a B&D densitometer read ing), then isn't more accurate to say that the B&W reading was more accurate? OK, I hope some of the more experienced photographers can enlighten me with this question before I run and buy a color densitometer.

That's why it is recommended that you use the blue channel of a densitometer. If you use a regular B&W densitometer, it reads only the silver image. The silver image is of a lower density since part of the density comes from the stain. By using the blue channel, you measure how much of blue light makes it through the negative (density + stain) since the stain effectively blocks blue light that the paper is sensitive to. Cheers, DJ.

AHHHH!! So I was understanding the opposite.....Thanks, that does make sense....:-))

I see my mistake now...I trnasposed the results, the B&W densitomer gave me a lower reading than the color, not the other way around like I stated in my original question. Thanks for your help it has made my life easier....

I have a Macbeth color densitometer. A friend of mine and I have been trying to plot curves and come up with development times using both PMK pyro and Rollo Pyro with HP5+ and other films. He's been doing much more work than I have but we've both concluded that, blue channel or no blue channel, densitometers and pyro just don't get along very well together. The readings are very inconsistent and also often come up with times that are completely wrong for normal, minus, and plus development. For example, one test will say that N is 6 minutes, another test on the same film will say it's 4 minutes, when in actuality observation indicates that it's really 7 minutes. So I would absolutely not waste any money buying a color densitometer to use with pyro.

I have had the same experience using a densitometer with PMK. It seems that the difference between PMK and, say, HC110 is not linear- ie you cannot assume that across different devloping times, exposure situations that a density of X with PMK corresponds to a density of Y with HC110. If it were linear we could just use a set of densities that correspond to different zones for PMK that would just be Z% lower than densities using HC110. I have always assumed this is due to the "compensating" nature of PMK at high exposure. I have found that a densitomter can be used effectively to determine effective film speed with PMK as the impact of stain at very low densities is minimal. For development times I have used step wedges and also a "standard" scene with several zones (my back yard)printed at min time for max black on a given paper/developer combo. Both work easily and give the same, reproducible, result.

Alan

wow, this is a relif! it is because of the same experiences that Brian and Alan had that I am having so much trouble, I could not seem to pinpoint accurate development times......Like Alan said I could determine film speed but not development. It almost makes me think that it is better not to use a color densitometer, maybe is better to just see how much silver density is present and try to "interpolate" to account for the increase in density due to stain. for example...if I know from my B&W densitomter that a density of .85 prints as Zone VIII then I can interpolate future readings. I think Alan has the best solution, is best to use step wedges .....thanks again...I am starting to see some light in this nagging problem.