Mark, keep in mind my test likely still has some amount of veiling lens flare (no matter how well coated the lens). Hopefully not too much. I've always wanted to run the same tests I've always done but by contacting transmission step tablets so that I could see how much flare is in my test setup. As Stephen Benskin would remind us all, contacting is the only way to eliminate flare entirely and get truly "raw" H&D curves.
You need to be contacting the wedge to the film.
There's nothing like reading Kodak's instructions! In their publication about B&W films, they say that if magenta remains, the fixer needs to be replaced and that the film should be re-fixed. I did that with my magenta films and the magenta cleared out. The surprise is: Density increased! Both TMY2 and Acros had retained magenta, and here are their graphs before and after re-fixing:
Attachment 62335 Attachment 62336
Re-fixed curves are red. I could see why removing dye would decrease fog and thus increase densities by the same amount, but these don't do that. Both of these look like the result of additional development. Densities are still too low, but I don't understand why re-fixing increased them at all.
BTW, when measuring the red TMY2 curve, I used many frames of different exposures of the step-wedge, so there is no run across either row on the step-wedge. Furthermore, I measured Acros the same way, and there is no slope-transition in it. That makes me suspect that the two slopes in TMY2 are really there after all.
You are still removing Dmin, right? The technically valid comparison would include Dmin.
Yes, the graphs are density above B+F. But including B+F would merely shift the curves upward; it would not change their shape and slope as we see above. That's what surprised me.
Originally Posted by Photo Engineer
I have seen curves with Dmin included in which the relative changes cause the curves to become identical when by removing fog they became different. And, since in printing or even just viewing a negative, we see the Dmin, this is a truly correct way of viewing the image in sensitometric curves.
I'm confused. If you mean Dmin as in zero exposure (ie base fog) how could the curve change shape by either including or excluding Dmin? What would explain that? Or am I out to lunch here...
Dmin is base + fog. Dmin is the term used for that by photo engineers just as they use Dmax for the maximum density in a curve.
Lets assume that we have 2 curves which are identical with base + fog removed. Imagine it. Now, add fog and the one with higher fog has higher Dmax as well. This is a 2 way street of deception. It can work the other way too.
The correct way to present a sensitometric curve is with base + fog or with Dmin shown. Sorry, but that is the fact here and the fact that removes any possible error in interpretation.
Ok understood. I thought you meant including or removing fb+fog density could change the shape of the curve (ie slope, curve shape etc).
I have to admit I was not aware it was correct practice to in fact plot gross density. I can see the value in that for a single film. But suppose one is comparing two films on the same graph. Since comparatively (ie from the perspective of the printing paper), Dmin must be printed through, why wouldn't we want to remove Dmin from the data? For example I know that Ilford's Delta 100 typically develops to Dmin of ~0.3 and Kodak's TMX develops to Dmin of ~0.2 (using my densitometer). Why wouldn't I "normalize" that out of the analysis?
The paper sees the base + fog, and so lets exaggerate this situation to 2 films with a Dmin of 0.2 and 1.0 respectively. Since they are vastly different, you would not print the foggy one at all. Subtracting base + fog might show that they have exactly the same curve shape though, so what might you think? I would probably discard the foggy film shot regardless of the match without for because the foggy example would take 3 stops more exposure to print through the film.
So, in this example, with a foggy film in one case, the example without B+F would appear to match the two curves, but in reality the foggy example is not printable.
Besides, if you slide curves around, as I noted earlier, then you can do the sliding for speed, curve shape, and B+F or minus B+F just by sliding the two sheets of paper.
I use a light table for this type of work.