How should I use a gray card when color-correcting scans in Photoshop?
Asked 7/24/2013
4 views
2 answers
0
I know how to use a Kodak gray scale card when photographing artwork to create a color-accurate reference. I’m now trying to do something similar for scanned images.
At school, we slightly defocused the gray card so it became a uniform tone before sampling it. For scans, I was thinking of selecting an area of the card in Photoshop and using blur or average to create a neutral patch for adjustment.
Is that a good approach for scanner color correction, and if so, how large should the sampled area be? Or is there a better way to get color-accurate scans?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
13y ago
2 Answers
2
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.
Originally by user1943. Source · Licensed CC BY-SA 4.0
user1943
13y ago
0
Generated from our catalog & community — verify before relying on it.
For scanner work, a gray card generally isn’t the best tool. A scanner has fixed, repeatable lighting, so the better approach is to calibrate the scanner itself with an IT8 calibration target/profile rather than trying to neutralize each scan from a gray patch.
If you want accurate, consistent color, proper scanner calibration is the key.
If you still want to sample a gray area in Photoshop, the exact blur/selection size is not critical. Just choose a reasonably large clean area that’s free of dust, stains, or discoloration, and average or blur it enough to avoid pixel-level texture. But this is more of a workaround than a real calibration method.
In short: for scans, use an IT8 target and scanner profiling; use gray-card sampling only as a rough correction method.
Recommended products
UniqueBot
AI13y ago
Your Answer
Related Questions
How should I set white balance from a gray card when the card shows different color casts across the frame?
How can I use a gray reference card to set white balance and brightness for scientific RAW color analysis?
How can I set accurate white, gray, and black points for product photos from RAW in Lightroom?
How does Lightroom’s gray-card white balance work, and why doesn’t subtracting RGB from 128 match it?
Why do different gray targets give different white balance readings?