# Sensitometry and fog

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• 09-12-2013, 09:55 AM
Michael R 1974
I agree I don't know how it could be measured. I'm just asking the question from a theoretical perspective.

Suppose one plots a ZS-style Net D/Log H curve with the usual goals of finding an EI speed point net D of 0.1 and Zone VIII net D of say 1.20. If the measured zero-exposure fb+fog density is 0.30, and one measures a Zone VIII gross D of 1.50, the assumption is that Zone VIII net D is 1.50-0.30 = 1.20. But what if at a gross D of 1.50, there is actually 0.35 fb+fog density? In other words, what if there is a >0 slope to the fog density "curve"?
• 09-12-2013, 10:18 AM
L Gebhardt
I don't know every factor that goes into building density on the film (I don't know if anyone truly understands it 100%). So any interaction between existing fog exposure and new light just falls into the area I don't fully understand. I'm sure the actual physics behind the exposure of silver salts are very complex. But from a sensitivity standpoint it's all just bundled together into a single density reading. Maybe this is responsible for some of the non linear responses we see, but I don't know.

Another way of saying it is: does it matter how the density got there, other than it was a result of light hitting the film? Either way the curve shows density vs light exposure. So unless you are interested in the physics of how density builds up it isn't important (not that it isn't important in an absolute sense).

I guess one test you could do is to look at the curves of film with low fog and the same film after aging with higher fog. If the curve shape shifted it may give you a hint that there is an effect. It does seem to cause a loss in film speed just based on casual observations.
• 09-12-2013, 10:25 AM
Bill Burk
This is where I think you might be interested in quantifying non-image density, for example infectious development, where silver halides that weren't exposed get developed, because the silver next to it was developed.

Maybe the question could be "Do some byproducts of development add density in the vicinity of the image?"
• 09-12-2013, 10:26 AM
Michael R 1974
Quote:

Originally Posted by L Gebhardt
Another way of saying it is: does it matter how the density got there, other than it was a result of light hitting the film? Either way the curve shows density vs light exposure. So unless you are interested in the physics of how density builds up it isn't important (not that it isn't important in an absolute sense).

I guess from a theoretical perspective (or even a practical perspective depending on magnitude) it might matter how the density got there if each incremental increase in density is not merely due to increasing exposure (light), but also increasing amounts of chemical fog.
• 09-12-2013, 11:57 AM
Photo Engineer
Fog actually has no meaning at exposures other than zero. But, OTOH, fog has no meaning except at the end of the full normal process cycle.

PE
• 09-12-2013, 12:12 PM
Michael R 1974
Is it then fair to say measurements for a particular film/developer based on sensitometry (such as ISO speed, contrast index, gamma etc.) are concerned strictly with absolute densities at exposure levels, rather than image or "useful" densities at exposure levels? In other words sensitometric measurements are concerned with the total density at any level of exposure regardless of how much incremental density is directly a result of the exposure?
• 09-12-2013, 12:17 PM
Mr Bill
I'm guessing that on a theoretical basis there ARE changes in the underlying fog, but for practical purposes, it doesn't matter; the only important thing is the resulting total density.

Let me make a hypothetical example. Say that 1) we have a system where development byproducts restrain further development, and 2) the unexposed areas have a slight tendency for unwanted development, increasing the base+fog level. My guess is that areas with greater exposure will release enough byproducts to restrain the "natural fog" in their immediate vicinity. So in this case, the "fog level" does not stay constant.

The real question is, to me, does this change anything about how I see or measure the density? I think the answer is no. Anything that I can measure, or that the printing paper can "see," has already included every effect and interaction, and I don't have any simple way to distinguish between any of these. Since neither I, nor my densitometer, nor my printing paper can tell these effects apart, I just treat them as though they are all effects of exposure. For my purposes, they effectively are. (Someone studying the mechanics of exposure and development would probably want to treat things differently, though.)

ps: I think the real reason one can subtract off the base+fog levels is that they essentially function the same as a neutral density filter sandwiched with the film.

(ps: the prior two posts came up while I was still writing)
• 09-12-2013, 12:31 PM
Bill Burk
Quote:

Originally Posted by Michael R 1974
Is it then fair to say measurements for a particular film/developer based on sensitometry (such as ISO speed, contrast index, gamma etc.) are concerned strictly with absolute densities at exposure levels, rather than image or "useful" densities at exposure levels? In other words sensitometric measurements are concerned with the total density at any level of exposure regardless of how much incremental density is directly a result of the exposure?

Well, I would be most interested in the signal density. The density I can use to make an image on a print.

So fog, due to light scatter inside the camera, has to be considered when it is part of the negative. So I might be looking at 1.2 density with 0.35 fog under it that I have to print through by doubling print exposure time. Despite the fact I have a low base+fog for that sheet. For example if my base+fog was 0.05 (an example of my B+F from 4x5 TMY2).
• 09-12-2013, 12:59 PM
Michael R 1974
But from the perspective of sensitometry, we're assuming no light scatter, right? No non-image forming light/flare. So what I'm really thinking about is chemical fog. "Signal density" is a pretty good term for useful density. I like that. Ideally that's the density we'd want to plot against log H. In Zone System testing, Net D at any exposure level (ie gross density minus constant fb+f measured at zero exposure) is assumed to = signal density. Probably good enough, but I just find it an interesting assumption. I guess there isn't really a way to isolate signal density from whatever the chemical base fog density is at any exposure level anyway.
• 09-12-2013, 01:09 PM
L Gebhardt
Quote:

Originally Posted by Michael R 1974
I guess there isn't really a way to isolate signal density from whatever the chemical base fog density is at any exposure level anyway.

From the perspective of sensitometry I would say there isn't a reason, even if there were a way. The sensitometry curves tell you exactly what you need to know about the film and development to make a negative tailored to make a print (well, except for the lens fog).
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