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I am familiar with the usage of a (Kodak) Gray Scale card when photographing, e.g. to get a color-proof replica of a painting. Now, I'd like to use such a card to ensure my scans are color-proof. At photo school, we slightly unfocused the card to achieve a homogenous color area.

What do you suggest is the best method to adjust that for scanned images in Photoshop. I'd tend to use the blur/average, but I wonder what's the best selection size to apply that effect.

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I would suggest not doing it this way. Instead, use an IT8 color calibration target and properly calibrate the scanner. Unlike in a regular photograph, the lighting doesn't change, so a consistent calibration is really what you need.

To answer the direct part of your question, if you do want to pick a part of the card to blur in order to get an even gray, just select a region that's big enough to target and which looks free of dirt or discoloration. It really doesn't matter much. But mostly, you don't need to do that.

A gray card in the field is useful because it gives you a reference for what neutral gray actually is under whatever color temperature of light is actually present. That's not the situation with a scanner. Although it's possible for the calibration to change slightly over a long period of time, basically you measure once and you're done.

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