# Calculating contrast index

Discussion in 'B&W: Film, Paper, Chemistry' started by lamda, Jan 5, 2010.

1. ### lamdaMember

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Now that I finally bought a densitometer, I am interested in calculating the contrast index, so I can see how my negatives compare with Kodak suggestions. (Don't worry, I will use other ways of evaluating them too, such as Adam's The Negative.) I found two formulas in the Kodak publication Monitoring and Troubleshooting KODAK Black-and-White Film Processes.

More accurate: CI = 0.128 + (0.267 x D-min) - (0.969 x TD) + (0.454 x LD) + (0.183 x HD) + (0.039 x D-max)

Less accurate: CI = (HD - TD) / 2.26 + 0.10

These, of course, are meant for use with Kodak process control strips, which I don't have. If I use an ordinary step pattern (21 steps) I figure D-min is the film base and D-max is any of the patches with such a strong exposure they are all the same. So which patches should I use for Toe Density, Low Density, and High Density?

Or, maybe there is a different formula to use with 21 step patterns.

2. ### ic-racerMember

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3. ### ic-racerMember

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Personally, I do mine on the computer and I use a linear regression to find the slope of the first 11 steps higher than 0.1 on the "Y" axis and use that as my "Personal Contrast Index."

All the myriad of methods for measuring contrast of the film have problems because some fiilms have a straight portion and some are totally "S" shaped or "J" shaped. Find a method that you like and stick with that, realizing that none are perfect.

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I would think that a plot of the empirical H-D curve would be much more interesting than an estimate of the CI.

5. ### RalphLambrechtSubscriber

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They are both interesting.

6. ### RalphLambrechtSubscriber

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That's a good book. I prefer the average gradient method, but CI is fine. Note that Ilford is using yet another method. To measure the CI yourself, I have attached two files. Print the first to plot your data. Print the second on clear overhead foil and use to measure the CI. make sure to print both at the same scale.

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7. ### lamdaMember

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Thanks, between the two of these I should be able to manage.