Patrick - I'm sorry if my comments upset you. That was not my intent.
Patrick wrote, "Are you also telling me that knowing the exact densities of a negative in any and all parts of the spectrum will somehow cause the paper to respond obediently?"
No. But it may lead us to making some measurements that may allow us to predict how the paper responds.
"We can use them to test theories or to make prints. Prints are seldom used to test theories, and theories do not always make good prints. Now I'm telling you something you already know."
There is an entire science called sensitometry that does this. And we are attemping to scratch the surface of it ourselves here.
"Let me reiterate something I think I said a long time ago, but maybe not in this forum. If a densitometer has a truly logarithmic response to input current, i.e. if its output follows the equation: Eout = k log(Iin) [...]"
I believe you did say this in this thread. And I even understand the math and electronics behind it.
"Nowhere in this derivation is there any requirement placed on the color of the illumination. If I measure in a part of the spectrum to which the photosensor is either more or less sensitive, I will find a shift in the calibration curve, but not a change in slope."
I'm not saying that your densitometer's slope will change with wavelength - otherwise it would be impossible to build things like spectrophotometers.
I'm saying the density of the materials you are trying to measure may change with wavelength. And sometimes significantly. That's why it could be important to control both the souce of illumination as well as the spectral response of your densitometer.
If your densitometer is responding to light from blue to near IR, without some sort of control of the light source and the sensor response, how will it be able to give us an accurate measure (say +/- 0.02D) of the density on a piece of stained film? How will it tell us about the UV absorption? I can't, as it is being influenced by all the other wavelengths that are striking the sensor - it is integrating the densities of all the wavelengths into one number. That is what my response to Donald's question was about.
And what about making measurements on a something that is obviously not neutral - say a piece of color transparency film. Without controlling the light source and the sensor, any measurements made would not be useful for much.
Patrick - I understand you are concerned about what wavelengths all these measurements are made at - it's a valid concern. That's why at this point, we should be using standardized materials for our density readings, things like Status filters or filters with specified properties, like the ones in Sandy's Gretag densitometer. At least that we we have some way to try and make comparable numbers. And there are probably better suited filters that could be used for stained negs, but until that has been determined, we must use what we have.
"In the first case, the H&D curve (I still prefer to honor the men who got us started) would have to be three dimensional, with the spectrum plotted at each point on the log relative illumination axis."
Yes, I've come to that conclusion as well. I really would like to see a graph of that...
And I've got no problem with honoring Messrs. H and D. - please do, we owe them a lot.
"Then we would know the spectral content of a pyro-developed piece of film. In the second case, we would want to know the spectral response of our printing material as well as the content of each part of the negative we want to print."
I believe that point is what got this entire thread started.
"If you are making contact prints by a printing-out process, I'll bet you will still lift up a corner now and again to see if it's done."
Yeah, that's why I don't like printing out papers...
Kirk