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  1. #1
    kaiyen's Avatar
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    gamma infinity - what is it and how to do it?

    Hi all,
    I know it's not called gamma infinity anymore. And I kinda know what it means in terms of it being a contrast curve and reaching an asymptotic point to...something. I'm not nec. looking for a hyper technical definition.

    But apparently that's not what it's called anymore, so I can't do an effective search. Can anyone help?

    My photo teacher talked about it the other day and it just flat out didn't make sense. He talked about putting Zone VII into II, increasing EI by a stop, which would be a pretty big underexposure of the shadows. I don't see how any development could fix that...but I'd like to know more.

    thanks,
    allan

  2. #2

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    Allan,

    The way I have seen the word used is in the context of the maximum contrast it is possible to get from a film. And it is primarily a question of the film's characteristics, not of developer choice or time of development, even though you can choose developers that get you to the maximum contrast faster than others. In other words, no matter how long you develop film, even with the most energetic developer, there comes a point when further development just increases overall density, but not approximate CI.

    Examples of films at the far edges of potential for contrast are Bergger BPF 200 and TMAX 400. Regardless of developer choice or time of develoment it is virtually impossible to get more approximate CI out of BPF than about .9. Tmax 400, on the other hand, will reach an approximate CI of well over 1.2.

    Sandy

    Quote Originally Posted by kaiyen
    Hi all,
    I know it's not called gamma infinity anymore. And I kinda know what it means in terms of it being a contrast curve and reaching an asymptotic point to...something. I'm not nec. looking for a hyper technical definition.

    But apparently that's not what it's called anymore, so I can't do an effective search. Can anyone help?

    My photo teacher talked about it the other day and it just flat out didn't make sense. He talked about putting Zone VII into II, increasing EI by a stop, which would be a pretty big underexposure of the shadows. I don't see how any development could fix that...but I'd like to know more.

    thanks,
    allan

  3. #3
    gainer's Avatar
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    The term "gamma infinity" was inspired by the shape of the curve of gamma vs time of development, which looks and is theoretically much like the curve of voltage vs time on a capacitor being charged through a fixed resistor from a constant voltage. This curve fits many film-developer combinations fairly well when there is a very small induction period. I have tried it on the data published by Kodak for XTOL and found a pretty good fit for all the films presented. It is easy to estimate a value of gamma infinity from two experimental values of gamma determined at the same temperature at development times such that one time is twice the other. Let C1 be the contrast index determined at T1 and C2 be that determined at T2 such that T2 is twice T1. Then :
    Cmax = C1 ^ 2 / (2*C1 - C2)
    The term C1 ^ 2 means C1 squared. * is the multiplication symbol. Cmax is the estimate of gamma infinity we are looking for. Once you have calculated Cmax you will find that Cmax, (Cmax - C1), and (Cmax - C2) vs development time lie on a straight line on semi-log graph paper. In fact, this line makes a nice chart for estimating the development time required to produce any other contrast index at the same temperature. You subtract the C. I. you want from Cmax and look it up on the graph. If it's higher than Cmax, you can't get it. Furthermore, lines for other temperatures pass through the same value of Cmax at 0 time. Thus, only one value of C. I. determined at a different temperature will give you a line for that temperature.

    How, you may ask, did I compare the equation with Kodak data when so few of the tables show values of C. I. at the requisite 2:1 development time ratio? I wrote a program to do an iterative (trial and error) solution for arbitrary developing time ratios.
    Gadget Gainer



 

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