How does Lightroom’s gray-card white balance work, and why doesn’t subtracting RGB from 128 match it?
Asked 7/28/2017
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I’m trying to understand how to correct color using a gray card. In one photo, the sampled gray card color is RGB 115,168,178. I assumed a neutral target should be 128,128,128, so I calculated a correction of -13,+40,+50 and applied it in Photoshop/GIMP. But the result does not match Lightroom’s custom white balance picker. What is the correct way to think about gray-card white balance, and why is Lightroom giving a different result?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
9y ago
2 Answers
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Three possible problems with your procedure.
- Neutral is NOT defined as 128,128,128. Neutral is equal RGB. 207,207,207 is Neutral. 69,69,69 is Neutral. Any equal RGB tones is Neutral (no color cast). And the 18% gray card is NOT 128,128,128. It is 18%, but gamma does raise it in our images, but it will still be a little lower than 128.
You are subtracting from 128, which is wrong concept. If you know the spot should be neutral, you should be trying to make the RGB be equal tones, whatever they are. Neutral is equal RGB, but not necessarily at 128. The difference from 128 is brightness (and is adjustable). The difference from equal RGB (of a neutral color) is a color cast.
The usual and normal and easy procedure is to use a White Balance tool to click on the color that we know should be neutral, and then the computer program makes it be neutral. Adobe Raw has such a WB tool, and the Photoshop Levels middle gray eyedropper does it as well.
18% gray cards are usually approximately neutral, but are not manufactured to insure neutral. They "work", but they are too dark to be best. Instead, they try to insure 18% reflectance, which is not your goal.
So instead, get a real actual White Balance card, made for the purpose. These are lighter gray color, and good ones are white. WhiBal is a very good brand card. I prefer the Porta Brace White Balance card ($5 B&H)
Originally by user38978. Source · Licensed CC BY-SA 4.0
user38978
9y ago
0
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Yes—your main assumption is the problem. A neutral gray is not defined as 128,128,128. Neutral simply means R, G, and B are equal. So 69,69,69, 128,128,128, and 207,207,207 are all neutral; they differ only in brightness.
That means you should not subtract values from 128 as a white-balance method. The goal is to remove the color cast so the sampled gray patch becomes equal RGB, while preserving its luminance as appropriate.
That’s why Lightroom’s white balance picker gives a different result: it is not just doing a simple per-channel subtraction toward 128. It likely performs the correction in another color model or with a more sophisticated transform so the chosen area becomes neutral without treating brightness and color as the same thing.
In practice, the normal workflow is simply to use the white-balance tool and click on the gray card (or another known neutral area). The picker adjusts the image so that area becomes neutral. If you want to reproduce this manually, think in terms of making the selected patch equal RGB—or neutral in a color space like Lab—not forcing it to mid-gray 128.
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