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  1. #11
    L Gebhardt's Avatar
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    I've never built a densitometer, but my RH Designs Alalyser has the capability to work as one. I've checked it against an X-Rite one I have and it is right on (relative values only).

  2. #12
    L Gebhardt's Avatar
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    Quote Originally Posted by Photo Engineer View Post
    The best method of fitting the characteristic curve comes from use of a cubic spline. Here is a reference:

    http://en.wikipedia.org/wiki/Spline_(mathematics))

    We use this in most all of our software, or at least did when I worked in the area.

    PE
    Thanks, that should be enough to get me started. Now I just need to figure out how to program it.

  3. #13

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    Quote Originally Posted by L Gebhardt View Post
    I've never built a densitometer, but my RH Designs Alalyser has the capability to work as one. I've checked it against an X-Rite one I have and it is right on (relative values only).
    I've no reason to believe that the aforementioned analyser is bad, but...

    1) I'm, ahem, cheap!
    2) I have a tight budget.
    3) I have an itch.

  4. #14
    L Gebhardt's Avatar
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    Quote Originally Posted by Anon Ymous View Post
    I've no reason to believe that the aforementioned analyser is bad, but...

    1) I'm, ahem, cheap!
    2) I have a tight budget.
    3) I have an itch.
    I should have expanded on this to say that the probe is a fairly simple looking thing and probably uses a cell like you mention. So, I'm sure it's possible. Just use the enlarger as a light source. You would need a calibrated step wedge to correlate the voltages to density.

  5. #15

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    Quote Originally Posted by L Gebhardt View Post
    You would need a calibrated step wedge to correlate the voltages to density.
    Maybe not. See post #10 in this thread.

  6. #16
    Photo Engineer's Avatar
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    You will find that Excel or most any spreadsheet program can make a quite good graph of sensitometric data. I have posted some of mine here.

    PE

  7. #17
    L Gebhardt's Avatar
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    Quote Originally Posted by Photo Engineer View Post
    You will find that Excel or most any spreadsheet program can make a quite good graph of sensitometric data. I have posted some of mine here.

    PE
    That's what I've found too. I also have the BZTS plotter program. But if I want to automate the process of reading a point off of a curve, say the exposure for 90% of DMAX I can't do that with Excel.

    I figure if all these program can nicely plot the line it shouldn't be too hard to get a nice function for each data set.

  8. #18
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    Tough one. First, you have to choose whether you are setting out to predict, or describe, what the film is doing.

    If you master the 'predictive' testing, you've made a lovely graph and cut off your creative future at your knees. Choose wisely. BTZS considered, it is just a beginning. Add imagination to the business of reproducing a perfect tone curve, and you will get photography, not numerology.

  9. #19
    Photo Engineer's Avatar
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    Well, you have to consider that many densitometers made nowdays have parallel and serial ports including USB. These can be connected to the computer to read the data, and a simple program can convert the data to row-column format for Excel. I've done this and it is fairly easy. I no longer have a copy of it, as it was written years ago (80s) and was used at Kodak for a number of chores including inverting row with column and sorting. This was before Excel or Lotus had those features.

    PE

  10. #20
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    For me getting the curve is not the problem. Solving a polynomial or cubic spline to get some useful data from it would be helpful. For example solving (X) for, say (Y=0.1) is the difficulty I have encountered.

    For example, I got a good 4th order polynomial fit with this Delta400 in T-max developer curve:
    f(x) = 5.480380E-2*x^4 + -2.520940E-1*x^3 + 3.262913E-1*x^2 + -6.918132E-1*x + 1.568020E+0

    But solving it for Y=0.1 is time consuming, even with a computer. I found it easier to just graph it and pick up any needed value off the graph.

    But if you were to write some software to easily solve a polynomial like this or a spline, that would be great. It would also be nice to have it solve for the X-intercept of a least-squares fit through the straight-line portion (another way to estimate speed as PE has pointed out in a related thread).

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