I am a little confused about density

Discussion in 'B&W: Film, Paper, Chemistry' started by lhalcong, May 2, 2013.

  1. lhalcong

    lhalcong Member

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    I have understood so far that density of 0.30 equals 1 stop. (same as saying 0.10 is 1/3 stop) . so if this is true for all fims (not sure), why is it that in my results (shown below) the numbers do not move in 0.30 increments for each zone. ?? They seem to be moving up in around approx ~0.15 (give or take) density increments, instead of 1 stop = 1 zone increment = 0.30 density. Why is that ??

    Notes: the Film was Tri-X 400 35mm format. Developer D76 1:1 process as close as possible to Kodak recommended settings. That is, agitation pattern, time, etc.
    I shot this test roll under sunny day in open shadow, light did not change as there were no clouds early in the afternoon, and camera meter matched the F16 rule... which gave me confidence that I was exposing somewhat correctly.

    Now, With small 21-stp stepwedge measured in exact same way, 1 stop increments does in fact match very closely to 0.30 increments in density... (I didn't post this results as it is not needed, assume true)
    This throws me off because I was assuming that to raise zone VIII to IX, I had to increase the density by 0.30 , but the numbers (column N) in my film as well as in Appendix 2 of AA box do not correlate to this. In fact. if recommended density for zone I is 0.10 and for zone V is 0.65-0.70 , there is only 0.55 of density between them. I was thinking each zone above zone I , I would have to add 0.30 that would add up to way past 0.65 recommended for zone V. Can someone clarify this for me. ? (by the way for this test column N-1 is -30% less time, and column N+1 is +30% more time).


    [TABLE="class: cms_table_grid, align: left"]
    [TR]
    [TD]Zone[/TD]
    [TD]N-1[/TD]
    [TD]N[/TD]
    [TD]N+1[/TD]
    [/TR]
    [TR]
    [TD]1[/TD]
    [TD]0.02[/TD]
    [TD]0.04[/TD]
    [TD]0.05[/TD]
    [/TR]
    [TR]
    [TD]2[/TD]
    [TD]0.08[/TD]
    [TD]0.14[/TD]
    [TD]0.17[/TD]
    [/TR]
    [TR]
    [TD]3[/TD]
    [TD]0.19[/TD]
    [TD]0.31[/TD]
    [TD]0.40[/TD]
    [/TR]
    [TR]
    [TD]4[/TD]
    [TD]0.32[/TD]
    [TD]0.53[/TD]
    [TD]0.63[/TD]
    [/TR]
    [TR]
    [TD]5[/TD]
    [TD]0.46[/TD]
    [TD]0.71[/TD]
    [TD]0.86[/TD]
    [/TR]
    [TR]
    [TD]6[/TD]
    [TD]0.58[/TD]
    [TD]0.88[/TD]
    [TD]1.09[/TD]
    [/TR]
    [TR]
    [TD]7[/TD]
    [TD]0.69[/TD]
    [TD]1.00[/TD]
    [TD]1.20[/TD]
    [/TR]
    [TR]
    [TD]8[/TD]
    [TD]0.83[/TD]
    [TD]1.19[/TD]
    [TD]1.41[/TD]
    [/TR]
    [TR]
    [TD]9[/TD]
    [TD]0.99[/TD]
    [TD]1.37[/TD]
    [TD]1.61[/TD]
    [/TR]
    [TR]
    [TD]10[/TD]
    [TD]1.12[/TD]
    [TD]1.54[/TD]
    [TD]1.80[/TD]
    [/TR]
    [/TABLE]



















    Thanks,
     
  2. Mike Wilde

    Mike Wilde Member

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    Research the HD curve. That will give you insight as to why parts of your results are not the linear output you are expecting.
     
  3. RPC

    RPC Member

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    The density change in the film will not be the same as the density change in exposure due to the gamma of the film. If your step wedge changes .3 density units from one gray patch to another, then the film density will change by the change in exposure (the step wedge change) multiplied by the gamma of the film. For example, if the gamma if the film is .5, then the film density change would be .3 x .5 or .15. So a change of exposure of .3 density units (one stop of exposure) results in a change of density on the film of .15 units. The actual gamma will depend on the film used, development time, and curve shape.
     
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  4. Gerald C Koch

    Gerald C Koch Member

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    Not completely true. A change in density of 0.3 equals 1 stop. Furthermore this is true only for a H&D curve that approximates a straight line with a slope of 0.45. Obviously such H&D curves are not straight lines over their entire length. The H&D curve is a semilogarithmic plot for which the log[SUB]10[/SUB]2 = 0.3.

    So what you espouse is a glittering generallity which is only partially true. As sugggested above read up on the H&D (characteristic) curve and things will be clearer.
     
  5. Michael R 1974

    Michael R 1974 Subscriber

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    lhalcong: I think you are getting confused between exposure and developed densities. It seems like you are using The Negative by Adams. Re-read the section on characteristic curves, exposure, density etc.

    Note that a step tablet/wedge is an exposure device for testing. It is not intended to tell you what the resulting densities of the developed film will be. Exposing film through a step tablet allows you to give the film known increments of exposure. That's all it is doing.

    Here are some easy to follow publications which may help to start you off:

    http://motion.kodak.com/motion/uplo...en_motion_education_sensitometry_workbook.pdf

    http://motion.kodak.com/motion/uplo...etters_filmEss_06_Characteristics_of_Film.pdf
     
  6. Shawn Dougherty

    Shawn Dougherty Subscriber

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    These are great publications which Micheal also recommended to me. I'm reading/working on them now and they are quite helpful.
     
  7. Alan Ross

    Alan Ross Member

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    Michael R is correct - you seem to be confusing exposure change with density change in processed film. 0.30 is the logarithm of 2, 0.06 is the logarithm of 4, etc.. Giving the film twice as much exposure normally would not result in twice the density on the film. John Schaefer, in his book Basic Techniques of Photography, An Ansel Adams Guide, Book 2, gives a completely lucid instruction on using a step-wedge film for testing AND plotting a curve for Zone System application. John is a fine photographer, PhD chemist, Trustee of the Ansel Adams Trust, and as President of the University of Arizona, founded the Center for Creative Photography in Tucson. A lot of other good info in his two books.
     
  8. Bill Burk

    Bill Burk Subscriber

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    You got good answers, 0.3 is for the x-axis. The y-axis is your result and it may vary...

    Think of what you are trying to achieve, this may make sense to you:

    For example, to "raise Zone VIII to IX" is one way of defining N+1. You basically say you like what you get when Zone IX is developed to N. "But this time, I don't have any Zone IX, my subject only goes up to Zone VIII"... "So I'll develop longer, until Zone VIII hits the same density I usually get for Zone IX."

    So look at the N curve, the density at Zone IX. Then look across left and right in 0.3 increments on the x-axis. Look for longer development time where Zone VIII hits the same density (That will become the N+1 time), and what shorter development time brings Zone X down to that density (That will become the N-1 time). And so on. If your curves don't hit the line, you can extrapolate the time.

    FYI Minor White instructed students to draw a "heavy horizontal line" across where N meets Zone VIII.
     
  9. lhalcong

    lhalcong Member

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    Indeed I was confusing the log of Exposure units with the density resulting from that exposure. I went back and read it again and thanks to all notes above I got it. Thank you all. I will read those books, I promise.

    Now going back to what I was trying to achieve which was determine my N+1 time to move Zone VII up to VIII and my N-1 to move zone VII down to VI . my N+1 time of 30% more development is correct. , however my N-1 -30% less development time has dropped the zone two steps which means 30% less was to low, I should cut it to about -15% . Is my reasoning correct ?

    thanks,
     
  10. Bill Burk

    Bill Burk Subscriber

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    Exactly.

    Your N and N+1 seem reasonable, but your data labeled N-1 graphs more like N-2. So the development time in between would be closer to N-1.

    I suspect your camera test is giving you curves that include flare. This makes it difficult to fit to curves and come up with N development times.

    If you have a curve family made of step wedge exposures that were contacted to film, you would have better developing time decisions.

    But for now, you can use the times you found here, they certainly are good enough.