U of S | Mailing List Archive | alt-photo-process-l | Re: Measuring DMAX

Re: Measuring DMAX

Stouffer's makes a Standard Reflection Step Tablet that goes up to Log 2.0 that can be used with a scanner to read density values up to Log 2.0.

Here is how you do it:

1.  Scan the standard step tablet as a raw scan—no adjustment, 16 bit grayscale.

2.  Set the black and white points in a levels adjustment layer and with the black point set on step 21 and the white point set on step 1

3.  Take readings on each step of the scan using rgb values and make a chart showing the RGB values and Stouffer's Reflection density of each step.

4.  Save the levels adjustment layer to disk for later use.

5.  Scan your print—as a raw scan with no adjustments, 16 bit grayscale.

6.  Load the levels adjustment layer you saved earlier.

7.  Take a reading for RGB level in the scan you just made.

8.  Find the RGB value you just read on your 21 step tablet chart and the log density value associated with it.

This method is pretty accurate....out to about +/- log .02.  Seems very consistent between scanners.

Best Wishes,
Mark Nelson

Precision Digital Negatives
PDNPrint Forum @ Yahoo Groups
Mark I. Nelson Photography

In a message dated 1/15/08 10:15:28 AM, gauvreau-yves@cgocable.ca writes:


In the limit you dont even need the equation, simply compare the raw pixel
level from a scan of the various process, relative values are plenty good
enough for this. If you need an absolute value then only a densitometer can
give it to you.


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