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  1. #1

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    Paper emulsion gamma values, please?

    Is it an acceptable approach to determine paper gamma around 1D output density (say, point 11 on X) like this: g=(Y12-Y10)/(X12-X10)?

    What are the values of gamma for different paper grades?

    UPD: point numbers are from Kodak Gray Tablet #2, 21 steps, starts 0.05D, step 0.15D. Steps are on X axis, Y is resulting print densities. Point 11 was printed to produce resulting density around 1 D, points 10 to 12 are on the linear segment of the resulting characteristic curve.

    Sorry for being unclear, my first post.
    Last edited by Julia; 01-20-2011 at 07:54 PM. Click to view previous post history.

  2. #2
    Mike Wilde's Avatar
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    I don't have the relevant texts at hand, but what you are describing rings true.

    Gamma in my memory is the slope (gradient, but I am not trying to tie it to the (G bar) that some densitometry texts talk about) of the straight line portion of the HD curve that you have generated (If you know you have made what is called an HD curve is another matter.) There is another graphics arts slope related measure called CI - contrast index. I am not as sure how it is determined, but it is more involved than the straight line slope meagire of gamma.

    Again, answering from work, I don't have the numbers at hand, but I think they are usually greater than unity.

    Gamma is not typically listed on the product packaging, or something most workers with b&w paper worry about.

    It is summarized in the technical information available from the manufactirers in most cases.

    It is typically shown graphically as a series of overlapping HD curves of a variable contrast paper exposed with different contrast filters.
    The slope (gamma) increases as paper filter numbers (contrast) increases.

    I hope this reply helps you in resolving your query

    Your post is a bit of an unusual first post.

    Usually the first post runs something like... I have been lurking here for a while, and have been shooting blank camera format for blank years....

    From your first post, you sound like you may be a photographer with an inquiring technical oriented and interesting approach to this craft/science called 'takin' picturs'.
    my real name, imagine that.

  3. #3

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    Thank you. Followed your advice and started some thorough search of relevant data sheets. Some further questions please.

    In one of the Kodak publications (November 2004 G-21 TECHNICAL DATA / BLACK-AND-WHITE PAPER KODAK PROFESSIONAL POLYCONTRAST III RC Paper) I found some data which I'm trying to understand.

    They speak of ISO Paper range, which they define as follows: "These numbers indicate the relative ISO ranges of different contrasts produced with KODAK POLYMAX Filters.The ranges were calculated from the log exposure ranges of the paper. You can use them as guides for selecting the appropriate paper contrast for the density range of a specific negative. When the ISO range of the paper approximately equals 100 times the density range of the negative, the contrast of the print will usually be satisfactory. (For convenience, the log exposure ranges have been multiplied by 100 so that the ISO ranges are expressed as whole numbers.) The contrast you choose will also depend on the nature of the subject."

    The table goes as following:
    contrast (paper) grade:
    -1 0 .5 1 1.5 2 2.5 3 3.5 4 4.5 5+
    ISO Paper range:
    >150 150 130 120 120 110 100 90 80 70 60 50

    From the description Kodak provides it looks to me like I need to divide the ISO Paper range by 100 and the result is the useful density range on the X axis. Is it so?

    Yes, I know I'm plotting H&D curves. I was not sure listing lurking time, cameras, or shooting experience makes an interesting introduction, but here goes. Lurked for a week or so, respect the place and read a lot of very interesting stuff on emulsion making. Topic camera now is Zeiss Ikon. Shooting experience with it is 1 year. Born in 1982 in Moscow, Russia. Coming from a family of three generations of photographers. Started shooting when I was about 10. Earning money with photography since 2001. Have a lot of digital stuff. Want to understand why I prefer film. It seems to me that film system is much more balanced, more following physics and physiology. I hope it answered some questions

  4. #4
    Mike Wilde's Avatar
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    Yes, ISO paper range is the relative of Gamma - How fast to I go from white to black is what ISO Range tells you - ie the slope of the HD curve for the paper.

    A thin negative (from underdevelopment) has a low slope, (because of lack of action time or lack of agitaion or excessive dilution or low temperature) The developer did not to convert (reduce) enough halogenated silver chloride/bromide/iodide to elemental silver, and the un- reduced halogantaed silver complexes were dissolved away in the fixing stage. So with low silver denisty, it has lower opacity i.e not a high enough range of densities. So the negatives gamma is low. It needs a higher than 'normal' gamma from the paper to yield a print with a hope at a convincing range of tones from white to black.

    The converse, with excess development action, builds too high a slope, in the negative, so the paper needs to have a lower slope to allow a range of tones from white to black to be rendered in the print.

    In the interst of brevity I am going to leave out under and over exposure effects, other than to say unless overexposure is extreme, it can usually yield a printable, but overall dense negative, and plausible looking print. The highlights in the print will end up compresed due to the negatives' densest parts of the image riding high on the curve where it begins to loose slope, but the developer action will still yield s good slope for the darker tones.

    If you are still curious, and not hung up on densitometry, David Vestal's 70's book 'The Craft of Photography' contains good examples of varying exposure and development and the effect on the final print. I more or less began a good part of what I started to teach myself about photography from what I learned in that book.

    It is nice to hear about your interest in flim. Perhaps it is somewhat genetic, or resurgent from what was once learned and forgotten from your earliest days.
    my real name, imagine that.

  5. #5
    RalphLambrecht's Avatar
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    Julia

    Papers are typically measured in exposure range and not gamma, but it can be done by dividing the 'useful' density range by the exposure range. See if the attached helps to answer your question, otherwise, feel free to ask.
    Attached Thumbnails Attached Thumbnails PaperContrast1.jpg  
    Regards

    Ralph W. Lambrecht
    www.darkroomagic.comrorrlambrec@ymail.com[/URL]
    www.waybeyondmonochrome.com

  6. #6
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    Jula;

    At Kodak, we measured gamma as the best straight line between a density of 0.8 and 1.2 density and being the ratio of Density to Log E. A grade 2 paper was roughly 2.5 using these terms. It varied a bit among the various products.

    PE

  7. #7

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    Mike, thank you, I think I read a lot on densitometry from what is available on the web - what I lack is experience in doing the exercise myself, and maybe some of what I read is not so accurate.

    Ralph, this illustration is familiar, I saw it in your other post; but probably glossed over it. It makes sense not to use gamma for the papers. It may be I have a wrong routine of film development. I develop to certain gamma value and thought if I would know the paper gamma it will speed up my paper selection in the darkroom. However in the course of experimentation I found a lot of fun to sort of reverse engineer the thinking behind emulsion making. I hope it will improve my understanding of how to shoot as shots end up printed and it seems that to select shooting parameters knowing how the scene will print provides better ROI.

    Ron, it is your combined film-paper-print graph that clicked on me. Because of it I exposed the paper so as to have #11 and 1D. Interestingly, the gamma I measured from the print is closer to 2.2 than to 2.5 you put on the graph. I measured the step tablet before making contacts from it and the densitometer readings confirmed the tablet is accurate.

    Thank you all, gentlemen. I really appreciate your explanations.

  8. #8
    RalphLambrecht's Avatar
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    Julia

    You are more than welcome. Do you have a copy of Phil Davis' book 'Beyond the Zone System'? If not, get a copy. It's a great resources and a fabulous stepping stone into sensitometry. However, never lose sight of the final goal: the image.
    Regards

    Ralph W. Lambrecht
    www.darkroomagic.comrorrlambrec@ymail.com[/URL]
    www.waybeyondmonochrome.com

  9. #9
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    The final desired gamma of the print is a function of the integral of the toe, mid scale and shoulder. This value should integrate to about 1.0 plus a little for camera flare and a fudge factor so that the human eye is "fooled" to see it like the original.

    PE

  10. #10

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    Ralph,

    Thank you for the recommendation. I looked at few pages available in the net and purchased the book.

    Ron,

    Thank you. I had a feeling that the integrated contrast should be close to linear, as we see it. Your formula makes it perfectly clear.

    As far as I understand the more contrasty is the paper the shorter is the input range, the usable range as you marked it on your graph. But even with a steepest curve we still have the toe and shoulder I think, is it right? From printing the tablet I can see that the lengths of the toe and shoulder change somehow, but not by very much. Does it sound right?

    I can also see that on a polycontrast paper the curve "breaks", is it because it is a mixture of two (three?) emulsions that start to be "density misaligned" in development if one goes to extreme contrast with filters?

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