Sensitometry and fog

Discussion in 'Exposure Discussion' started by michael_r, Sep 11, 2013.

  1. michael_r

    michael_r Subscriber

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    This might just be a question for PE, but figured I'd throw it out there anyway. Given a specific film, hold time, developer, CI etc. etc., how do we know base fog (measured at zero exposure) is constant/fixed as exposure/density increases?

    Michael
     
  2. Bill Burk

    Bill Burk Subscriber

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    Because anytime you develop unexposed film for a specified period of time, the base + fog doesn't change.

    It's when you develop less/more time, or expose the film in a way that can affect the entire sheet, that you start to see changes in base + fog, right?
     
  3. Mr Bill

    Mr Bill Member

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    Hi, I'm not sure that it makes any difference, as the total density affects any use that I can see.

    I think it's clear that that a sub-visible exposure WILL affect the result of any following exposure, as this IS equivalent to "flashing." I don't think there is any simple way to know that this is going on, so the rule is to be sure no significant light is present during the entire process.

    I know that you must have had a reason for asking, if you care to share.
     
  4. Bill Burk

    Bill Burk Subscriber

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    I wonder too... Maybe my answer didn't help your question.

    Perhaps you are wondering if ALL the density that accumulates for exposures above zero, is above your baseline base+fog solely because of the exposure. (All image forming exposure)

    Or maybe you are wondering if the base+fog might grow due to increased developer activity because the exposure caused increased local development. (Some infectious development effect)
     
  5. ic-racer

    ic-racer Member

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    This could be the start of a long thread. My empiric data suggest that base fog in old film does not improve sensitivity (like flashing or pre-exposure might). In fact all the expired/fogged film I have tested the fog seems to eat away at the toe and make the film slower.

    However, fog level is also dependent on degree and type of development. So, one might wonder about a very active developer used to produce super speed but which also gives a high dense fog level. Would the high fog negate any speed benefit of super-active development? Apparently one CAN have very high fog and increased speed as in this example:

    Highfogandmorespeed.jpg

    Dotted vertical line is the fractional gradient speed point. Delay is the time from mixing the developer; it lost activity quickly after mixing, so time-zero gave both the highest speed and highest fog.
    JOURNAL OF THE OPTICAL SOCIETY OF AMERICA
    An Evaluation of Film Speeds Obtained with Kodak SD-19A Developer
    P. HARIHARAN
     
    Last edited by a moderator: Sep 12, 2013
  6. Gerald C Koch

    Gerald C Koch Member

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    An interesting question. Of course the model sets emulsion fog as a constant. This is in contrast to developer fog which increases with development. If the film is exposed to create an image and the film is not developed the activation sites (latent specks) will migrate about destroying the image and resulting in an overall increase in fog. This is ensured by the Second Law of Thermodynamics. But this would take a long time. The time would be shortened by higher temperatures. This is why exposed film should be kept cool before development.
     
  7. michael_r

    michael_r Subscriber

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    Thanks everyone for responding so far. Interesting points. Where I'm coming from on this - we have an implicit (or explicit) assumption in any sort of sensitometric study of a film/developer combination, whether it be ISO, Zone System etc. that whatever we measure as base fog density can be subtracted from all exposure densities to determine net density (or image density). In other words, suppose you plot a characteristic curve for a film. Base fog density is assumed to be constant all the way across. In a Zone System-style test, we are usually told explicitly to subtract this base fog density from all exposure densities. In the ISO triangle, it is not an explicit procedure because we are concerned with low densities above base fog (speed point) and an increase of 0.8 above m at an exposure 1.3 log H higher. But there is still an implicit assumption the delta D of 0.8 is "usable" or "imagewise" (pick your term) density, meaning fog is assumed to be constant.

    Said another way, we know base fog (zero exposure) for a given film type can sometimes vary with age, hold time, developer and development CI. Can it also vary with exposure?

    Example - suppose you do a sensitometric study of FP4 developer in ID-11. You measure fb+fog density to be 0.30. Choose any other exposure value on the curve, say one that produced a gross density of 1.50. Is the base+fog density still 0.30 at that gross density? Are we correct in assuming net density at that point is 1.20? Then change the developer to something with different developing agents and/or a stronger alkali and repeat. What then?

    The emulsion could be a variable, and the developer could be also. We know certain developing agents and/or formulas have tendencies to "discriminate" better than others when it comes to exposed vs unexposed silver halide grains. Is it possible in areas of high exposure where lots of reduction is occuring, more unexposed grains might be "infectiously" developed?

    Granted, even if this does happen, the differences are probably very small, and I don't know how you could measure it anyway. So yes this is probably unimportant in the grand scheme of things, but I'm still curious. PE once told me (if I'm remembering this correctly) that when they tested films etc. at Kodak they always plotted gross density.
     
  8. L Gebhardt

    L Gebhardt Subscriber

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    Does it matter? If the fog somehow contributed to an increase or decrease in a way proportional to exposure it's just another factor in the curve. In effect by measuring the curve you are taking all such effects into account (if they exist at all).
     
  9. michael_r

    michael_r Subscriber

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    It doesn't matter because the growth of fog with increasing density is probably small (if it exists at all). But I don't agree this effect (again, if it even exists) is taken into account when the curve is plotted. If the plotted curve represents gross density, then fog is included everywhere. No problem there. But characteristic curves are very commonly plotted showing net density, where a constant fb+fog density is subtracted from all the measured densities. People calculate contrast, etc. this way.
     
  10. L Gebhardt

    L Gebhardt Subscriber

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    But if you subtract the fb+fog based on a reading of the clear area from all the density and variation on the fog based on density will still be left in the curve. So I don't see how it matters with net or gross density regarding any theoretical fog variation to exposure. If fact I'm not sure how your ever be able to figure out if this effect exists since all we can measure is the final density in response to exposure.
     
  11. michael_r

    michael_r Subscriber

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    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"?
     
  12. L Gebhardt

    L Gebhardt Subscriber

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    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.
     
  13. Bill Burk

    Bill Burk Subscriber

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    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?"
     
  14. michael_r

    michael_r Subscriber

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    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.
     
  15. Photo Engineer

    Photo Engineer Subscriber

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    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
     
  16. michael_r

    michael_r Subscriber

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    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?
     
  17. Mr Bill

    Mr Bill Member

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    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)
     
  18. Bill Burk

    Bill Burk Subscriber

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    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).
     
  19. michael_r

    michael_r Subscriber

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    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.
     
  20. L Gebhardt

    L Gebhardt Subscriber

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    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).
     
  21. Photo Engineer

    Photo Engineer Subscriber

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    Well, given a step wedge and an image on the same film and processed the same way, any fog is present in both and is "used" to produce any image from the negative. So, what you measure and see are the same. The difference is that your eye integrates the entire image whereas the densitometer does not.

    The integration is the major difference between what you see and what you measure, and not the fog.

    PE
     
  22. michael_r

    michael_r Subscriber

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    I'm not sure I follow
     
  23. Photo Engineer

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    Your eye sees the entire picture and judges contrast, the densitometer sees one step at a time and measures contrast. These two pieces of data do not need to correspond. Thus, fog is different to you as the observer and the curve that might be drawn.

    In making enlargements or contact prints, fog tends to lower contrast even though it is uniform throughout. That is because it lowers the difference between Dmax and Dmin. However, by use of the right print contrast, this can be removed.

    PE
     
  24. Prof_Pixel

    Prof_Pixel Member

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    Think of it in terms of the signal to noise (S/N) model used in electronics. The fog is the 'noise' and the negative density represents the 'signal + noise'. When the 'signal' value is low, the 'noise' value becomes significant; when the 'signal' is high, the noise is insignificant. The result of the fog ('noise') in photography is to lower the image contrast (as Ron has said).
     
  25. michael_r

    michael_r Subscriber

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    I thought I would revive this thread with a somewhat cryptic note in Haist's sensitometry chapter regarding D-min (fog) in relation to the plotting of a characteristic curve, and what made me start to think about this and theoretical implications to ISO testing etc.

    "In some cases the D-min has been subtracted from all densities, making the D-log E (H) curve touch the line of zero density."

    As we know this is a common practice when people perform various types of Zone System tests for personal EI and contrast. It is advocated by Adams etc. The next sentence in Haist is:

    "This practice, however, fails to take into account that fog silver is not distributed uniformly over all levels of image density."

    Nothing more is said, but it got me thinking. The footnotes to this sentence reference two sources which may or may not have any relevance today:

    R. B. Wilsey, "Fog Corrections in Photographic Densities," Phot. J., 65: 454 (1925)

    H. A. Pritchard, "The Fog Correction of Photographic Densities: A Sensitometric Study," Phot. J., 67: 447 (1927)
     
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