Why can bright light make preset white balance fail or give clipped results?
Asked 7/12/2020
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I’m using a Nikon D5200 and trying to set preset white balance in bright, diffuse daylight. Sometimes the camera shows “unable to measure white balance,” which the manual says can happen if the scene is too dark or too bright. I also tried photographing a white object and using that, but the result was heavily clipped. When I reduced the exposure by stopping down the lens, the camera was able to measure white balance and the results looked better.
Why does very bright light prevent white balance measurement, and why does using an overexposed white target lead to bad white balance? What should I use instead when setting a custom/preset white balance?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
6y ago
2 Answers
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White-Balance correction works best with a bright grey. Most grey surfaces work but precision is lower when the surface is darker. Correction will not work if any of the channels are clipped which is why too bright is a problem.
The reason extreme brightness is problematic is that the camera is unable to figure out what transformation to apply if the information is not accurate (dark) and incomplete (excessively bright). You can get a better idea of the issue with a simplistic example using a linear color-space for simplicity:
- Say the grey color patch shows RGB=165,155,245 at some exposure.
- The same patch would show RGB=84,72,122 one stop lower which loses some precision
- Every stop darker you would lose precision since on a digital camera each stop has one fewer bit-depth
- A stop brighter through would give RGB=255,255,255 because all channels would get clipped.
With clipped channel from an overexposed image, there is no way the camera can deduce what factors it needs to apply in order to correct for white-balance. With darker images, it will compute some values but their result is less precise because there are fewer bits to represent nuances in lower stops.
The numbers above are greatly oversimplified since cameras can perform the measure and computation at the sensor bit-depth, often 12-bits or 14-bits, before doing the rendering to JPEG but this makes the illustration easy to understand, hopefully.
Originally by user1620. Source · Licensed CC BY-SA 4.0
user1620
6y ago
0
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Preset white balance needs a neutral target with usable RGB data. If the target is too bright, one or more color channels can clip, so the camera no longer has complete color information and can’t reliably calculate the correction. That’s why very bright light can cause “unable to measure white balance.”
Using a white object makes this worse: true white is already near the maximum recordable value, so it is easy to overexpose it. Once values clip, the camera cannot tell what color cast was present before clipping.
For custom WB, use a neutral gray target (commonly an 18% gray card), not a white object, and make it fill the frame. Focus does not matter; coverage does. Expose for the card itself so it is bright but not clipped. If the light is still too bright at your lowest ISO and smallest practical aperture, reduce exposure further or use a darker gray target.
In short: bright light doesn’t directly break white balance; overexposure and clipped channels do.
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