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  1. #1
    holmburgers's Avatar
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    A Theoretical Quandary - Printing Out Paper

    Hello good people,

    I have a theoretical question that is perplexing us at George Eastman House, regarding the contrast effect obtained with printing out paper. In this case, collodio chloride POP.

    Here is the mystery: By slowing down the printing time, higher contrast is achieved.

    A low contrast negative is printed in open shade or under diffusion, whereas a high contrast negative is printed under direct sunlight (historically). However, the diffusion aspect is not the culprit (and besides, wouldn't this tend towards lower contrast anyways?), because the same effect is obtained with a yellowed glass. The key here is slower printing, with light of less intensity.

    This has been known for a long time, and is considered a standard control in the process. The reason for WHY however, has alluded us.

    Some things to know about POP paper. It is pure silver chloride with an excess of silver. As the image prints, it is "self-masking" (that is, metallic silver is formed and thus blocks light from penetrating further).

    Now why on Earth is this the case? If the negative has a fixed set of densities from high to low; why would only printing time, or rather the intensity of light, affect the outcome. Could this be due to some kind of reciprocity phenomenon? Could this be due to particle size, and in some way affected by the self masking quality of POP?

    I'm interested to hear your ideas, and intelligent thoughts on the matter.

    Cheers!
    If you are the big tree, we are the small axe

  2. #2
    Ian Grant's Avatar
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    This is the same with Albumen prints as well. You use tissue to reduce exposure and increase the contrast (I went on a course along with another APUG member this summer, the tutor is one of Mark Osterman's former wet plate students - small world)

    It must have something to do with the self masking effects of the emulsion during exposue and the threshold inertia needed, so the fast that mask builds up the lower the contrast. So yes akin to reciprocity.

    It happens with colour papers, the owner of my local Pro lab was an ex Durst technician and once told me that on the old roll head printers the lenses were set to give identical exposures regardless of print size (enlargement is done by using different focal lenght lenses rather than distance), this prevent contrast shifts and also colour shifts due to reciprocity.

    Ian

  3. #3
    Bill Burk's Avatar
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    Is it the Callier effect operating in reverse? In direct sun a high-contrast neg will block the shadows quickly reaching their dMax. But In open shade, the Callier scattering, will allow some scattered light to slip in under the metallic printed silver to make them denser.

  4. #4
    Bill Burk's Avatar
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    But I don't know how the yellowed glass would play with that...

  5. #5
    ic-racer's Avatar
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    Doesn't seem any different than using B&W negative film. The dim light needed for longer exposures doesn't produce density in the dimmest areas of the image (reciprocity failure).

  6. #6
    Photo Engineer's Avatar
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    Might it be the contrast reciprocity effect?

    PE

  7. #7
    Klainmeister's Avatar
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    I have seen this phenomenon before and thus tested an image when high intensity was used briefly, waited, then did another burst of high intensity. No change vs doing a single high intensity. If you do a gradual increase in intensity, it acts exactly the same as if you used diffused light throughout, but the reverse produces the same as high intensity.

    I think the answer lies in the self-masking effects of a POP system, where that first burst of light creates the mask and you end up with your given contrast based thereon.
    K.S. Klain

  8. #8
    holmburgers's Avatar
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    Hey guys, thanks for weighing in on this.

    The more I've thought about it the more I think the whole(?) phenomenon can be explained with reciprocity failure. Each successively more dense part of a negative is going to experience more 'reciprocity failure', and if the overall intensity of light is reduced this effect will be exaggerated throughout the scale of densities (thus, more contrast).

    The masking explanation doesn't make sense to me... and that's not to say it's wrong, but I just can't reason it out myself. My thinking is that the printed out silver which constitutes a mask is going to be formed at the exact moment the silver-chloride achieves enough exposure and that's that. You can't sneak in any silver ahead of, or behind exposure. The mask is built up gradually just like the exposure is; in fact it's created in direct proportion to exposure, it is after all our only evidence of exposure. This doesn't belie the reciprocity failure explanation (the disproportionate relationship of light to exposure). It's impossible however, for exposure and deposited silver to diverge from one another, they're 1 in the same, and if that's the case, how can the masking have an effect on contrast between 2 different exposures of the same negative?

    Though... Klanmeister... that's an interesting caveat... it does kind of make sense how an initial intense exposure could set you up for a given contrast, because this would lay down a mask which doesn't show the effects of reciprocity, and that would undoubtedly have an effect on the subsequent printing. Hmmmm...

    The plot thickens!

    What do you guys think?
    If you are the big tree, we are the small axe

  9. #9
    Klainmeister's Avatar
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    I'd love to hear what you find out--this is from a simple experiment performed about a year ago because I had found the same phenomenon and was trying to pin down contrast. That said, I never ruled out reciprocity, but also didn't take the time to experiment further.

    While you are right about the masking not making sense as a response to light intensity....there are many things that we have experimented with recently that haven't made too much sense. We're currently working on a new Carbon tissue and it's the opposite of what physics is telling us. Likewise, try adding more PT to a PT print and you get less contrast, less dynamic range, weaker blacks...but there's more metal!

    Anyways, please let us know what you find. Hope you haven't froze your jiblets off already up there in the great white east.
    K.S. Klain

  10. #10
    Bill Burk's Avatar
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    Yes, the failure of the Bunsen Roscoe reciprocity law.

    I bet you could measure this effect and use the results of the study to devise a set of variable-contrast "yellow" filters you could use in direct sun (or a UV printer)...

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