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  1. #11
    Helen B's Avatar
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    "How do you generate CI?"

    The simple, approximate version of CI is the angle of a line between

    - the point on the curve that is 0.1 density above FB+F, and

    - the point on the curve with an exposure of 2.0 greater than the first point.

    But this isn't exactly correct: the points are really on arcs of 0.2 and 2.2 scale units (x and y axis drawn to same scale) with a centre at the intersection of the horizontal FB+F line and the straight line intersecting the two points. Most easily found by using a little ruler with marks at 0.0, 0.2 and 2.2 units and placing it on the curve so that the 0.0 mark is on the FB+F line and the other two marks are on the curve; or in software by iteration based on the initial value given by the simple method.


    Best,
    Helen

  2. #12

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    Quote Originally Posted by fhovie
    How do you generate CI? if the SBR is 5 and the DR is 1.25 - what is the CI? and how do you get it - also - I know from my step wedge that my grade 2 paper can print a DR of 1.25 - What if I do a kalitype - I need a DR of 1.75? I am always reading about CI differences from AZO to Silver to Alt process. How do you calculate CI?
    If you are doing the BTZS why dont you just go with Phil's average Gamma?
    In any case you can approximate CI values with the formula.

    CI=DR/SBR.

    In real practice you need to make a "ruler" marked at log lenghts of 0.2 and 2.2 and keeping the 0 point at the b+f line you adjust the ruler until the curve touches both the 0.2 and 2.2 points. You can then calaculate the slope from these two points. Of course for this to work you have to havew the ruler as well as the coordinates for the curve with the same scale.

  3. #13
    fhovie's Avatar
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    So AZO (1.7 DR) CI would be higher than Grade 2 (1.25 DR) Silver CI - with an SBR of 5: DR/SBR 1.7/5 (.34) vs 1.2/5 (.24)

    CI does seem rather complicated to me - I would think that DR would be plenty useful enough - I know what my paper ES is and the trick (I think) is to match what I want the SBR to translate to in DR so I can get the shadows and the sparkle on my favorite paper without highlight blowout or muddy shadows. What is the advantage of knowing CI?
    My photos are always without all that distracting color ...

  4. #14

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    Quote Originally Posted by fhovie
    So AZO (1.7 DR) CI would be higher than Grade 2 (1.25 DR) Silver CI - with an SBR of 5: DR/SBR 1.7/5 (.34) vs 1.2/5 (.24)

    CI does seem rather complicated to me - I would think that DR would be plenty useful enough - I know what my paper ES is and the trick (I think) is to match what I want the SBR to translate to in DR so I can get the shadows and the sparkle on my favorite paper without highlight blowout or muddy shadows. What is the advantage of knowing CI?
    You are mixing terms, Azo has an exposure scale not a DR, for purposes of fitting the negative to the paper, we equate the negative DR to the paper ES. So to answer your question, yes you find out the paper ES and make the negative DR equal to this. The CI or gamma or whatever you want to use, is so that we all talk the same language and can use your data in our personal work. If you say I use TMY to a CI of .87 to print on azo grade 2, I know exactly what to do in case I want to print in azo.

    Many times as you say, people specify a DR for certain processes, but if we are to give all the information then Gamma or CI are more accurate.

    From what I have seen of your posts, you seem to have your process very well controlled, so I would say just keep on doing what you are doing...

  5. #15

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    Jorge, I think you meant to say average gradient not average gamma.

  6. #16
    Aggie's Avatar
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    Ya'll make my blonde roots hurt. I do not want to revert back to kinematics to figure out how to take a purty picture!
    Non Digital Diva

  7. #17

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    Quote Originally Posted by Claire Senft
    Jorge, I think you meant to say average gradient not average gamma.
    Claire, I meant Gamma and CI as explained in the literature. Phil uses average gradient as his choice for calculating the slope which is very similar to Kodak's CI. BTW, I made a mistake, Ilford uses average gradient instead of gamma as I stated in the previous post.

  8. #18
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    Well - they used to be blond - my kids (and CI) turned many of them grey! -

    I hope to never see a density of 2.2 unless I do it as a test - so in the normal evaluation of exposing a scene and printing, CI is not very useful (I think) I just go after ES and DR and all should be good - (Both 1.25 for my Grade 2 papers)
    Now - in evaluating developers (Which I am often doing) If I shoot 12 frames of a grey card metered one below zone 1 -each fram one stop up - and finishing with one above zone 10 and develop it - I should have a pretty good idea of how a developing process effects film speed and contrast expansion and compression. I should see zone 1 at B+F +.1 - which is exactly what I need to know to shoot a purty picture. It is a good thing I was able to bag 30 rolls of TMY cheap! The last batch of developer should be good for 200 rolls - I will know my process by the end of the week - I just needed good starting points.
    My photos are always without all that distracting color ...

  9. #19

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    Jorge, my mistake then. I thought Mr. Davis was using the term average gradient, not average gamma. Thank you for correcting me.

  10. #20

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    Quote Originally Posted by Claire Senft
    Jorge, my mistake then. I thought Mr. Davis was using the term average gradient, not average gamma. Thank you for correcting me.
    You are correct, Phil uses average gradient, but I was talking about plain gamma and CI. Nothing to correct here....

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