Originally posted by BrianR WB changes the jpeg preview, which then changes the exposure histogram. The exposure histogram does not appear to be independent of the individual colour channels (on the k5iis at least).
Have you tried two identical raw shots with drastically different WB? Below is what you'll get (k5iis aimed at a solid green target). The exposure histogram changes a good deal (not as much as the RB arts of the RGB of course
). If I throw these two raw files into lightroom and change their WB to something common, the histograms then match each other in lightroom.
Same raw file except different WB setting, different exposure histogram on the back of the camera. The exposure histogram is not based solely on the raw data captured.
You can try a similar experiment changing the saturation, contrast, etc. jpeg settings while still shooting in raw.
I see what you posted, and can agree that changing the WB can change the histogram. That is of course because you have radically changed a color channel by putting the WB dramatically off target. In essence you are trying to force the shade (lightness & darkness ergo exposure) of the color to be something it is not. The overwhelming majority of photographers don't go about shifting their WB to such extremes from real world color.
When I said independently, I was talking about in terms of photo creation. I don't know of any photographer that is thinking about the separate color channels having to be just so to get a proper exposure. Generally one tackles getting the WB colors right first, then they approach adjusting the exposure as necessary to get it correct. Mentally they treat the two things independently.
Try this experiment. Take a picture of a plain white surface. Increase your compensation so that it appears the same brightness as -you- see it. Now set your camera to Manual mode to the same settings as the last shot including WB, and take the picture again but as a JPEG. Then, using all the same settings including the WB, take the shot again but in RAW. You will notice that the histograms match.
I have never encountered any other photogs that regard WB and exposure as being related. They treat them as independent entities.
As for exposure histogram accuracy, as long as you have strived for accurate color, then whether you believe the exposure histogram is JPEG or RAW based won't really matter, because it still will lead you to draw the same conclusions about what is going on with the image exposure-wise.