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brucep
1-Jun-2015, 06:35
I can measure the Dynamic Range of a negative by a Densitometer, but ideal negatives for different processes are often given in CI.

I'm sure that this is basic trigonometry, but it has been a while since school days, so can someone explain how to change from one reading to the other?

Thanks
Bruce

ic-racer
1-Jun-2015, 14:15
A reading of a negative gives you the highest and lowest "Y" values (Dynamic range). You need to know how much light hit the film during exposure for those same areas to get the "X" values in order to make the triangle to estimate a CI (Contrast Index). If you took notes back when you exposed the negative, you can plot the X values as your "Zones" you got with your meter for those same areas of the processed negative you are measuring with the densitometer.

Example:
A) Shadow under a rock, measured on the negative 0.25 log D. Exposed at zone II
B) Highlight of the rock in sun, measured on negative as 1.35 log D. Exposed in camera at Zone VII

Your Dynamic range is 1.10 log D
Your estimated CI is (1.10/0.3) / 5) = 0.73

ic-racer
1-Jun-2015, 14:18
Just to be more clear on that last bit of math for the triangle: 1.10 divided by 0.3 converts your dynamic range from log d into STOPS on the 'Y' axis. Zone II and Zone VII are 5 stops apart on the 'X' axis.

ic-racer
1-Jun-2015, 14:25
Since your Contrast Index is independent of any scene, you can just measure it at home in the darkroom. Contact print a step wedge to the film. Process the film with the same time and developer as your other pictorial images. Read the step wedge and the resulting negative patches with the same densitometer and and plot them out on graph paper to get the slope (CI).

Bill Burk
1-Jun-2015, 15:53
Contrast Index is a measure of the slope between two certain points on the curve. You pick the points in a way that measures the best part of the curve, the part of the curve you are probably going to use when you print.

There is a paper discussing it ... Papers (http://beefalobill.com/benskin/)

But if you want to skip the math and get right down to it... Print out the graph paper, plot your curves, and lay the Contrast Index Meter on top of it...

Sensitometry Graph Paper (http://www.beefalobill.com/imgs/sensitometry.pdf)

Contrast Index Meter (http://www.beefalobill.com/images/cntrastindexmeter.pdf)