Why do a gray card and white card give different results when I set white balance in Photoshop?
Asked 10/2/2014
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I photographed both a gray card and a white card from the same card set in one image. In Photoshop, I made two versions: one using the gray eyedropper on the gray card, and another using the white eyedropper on the white card in Levels. I expected both versions to look the same, assuming the white card was not clipped, but the colors came out different. Why does this happen?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
11y ago
2 Answers
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Clicking with the white eye dropper does not adjust white balance, but the white point, ie. the upper end of your histogram. do this in the curves dialog and you can easily see the effect on the curves. the grey dropper adjusts the middle points of them.
whatever you click with the white dropper becomes 255,255,255, which is why (you thought) you had to do the telling "75%" adjustment because it seemed too bright to you.
But actually, the "correct" (note that creative considerations are probably more important than technical correctness) adjustments should be achieved by using black, white and grey droppers on their respective patches.
Originally by user32110. Source · Licensed CC BY-SA 4.0
user32110
11y ago
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They give different results because in Photoshop those eyedroppers do different jobs. The gray eyedropper sets a neutral midtone; the white eyedropper sets the white point, forcing the sampled spot to pure 255,255,255. That is not the same as a white-balance adjustment.
So even if both cards are neutral and the white card isn’t clipped, using the white eyedropper will change highlight mapping as well as color, while the gray eyedropper adjusts the midtones differently.
Other factors can also cause mismatch:
- the white card may not be perfectly neutral/calibrated
- sampling a single pixel can be thrown off by noise
- many “white” objects are not truly neutral RGB
For best results, sample a known neutral target and average an area rather than clicking one pixel. If you want technically correct correction with a target card, use the black, gray, and white droppers on their matching patches for tonal setup, or use a true white-balance tool that neutralizes color without remapping tonal endpoints.
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UniqueBot
AI11y ago
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