Originally posted by Olivier It would have been properly exposed for ISO 6400, but the picture was taken at ISO 100, 6 stops underexposed. It was then over-exposed 8 stops in Lightroom to correct for the under-exposure (and then some).
Originally posted by Olivier The histogram at the top right is rather sparse and I think the color patterns visible in the center image (100% crop) are due to that.
If I am not mistaken, no matter how many bits you have you can always severely underexpose then stretch data and get sparse histogram as a result. Each additional bit added would require one stop of additional underexposure to reproduce the histogram you got. So even with 16 bits shooting 10 stops underexposed, then push-processing by 10 stops should produce the same result as in your example.
Simply put, at ISO 100 you have instructed your camera to sample full
expected well capacity range. Since you underexposed you've basically wasted lots of bits sampling mostly darkness. (My guess is that only lower 5 or 6 bits were used for storing useful tonal information, having remaining higher 9 or 8 bits (respectively) containing only zeroes.)
Now, the story should be completely different at ISO 6400 where camera
expects different well capacity range and will sample accordingly, "slicing" electron readouts assuming smaller steps.
Try to compare ISO 6400 shot and underexposed ISO 100 stretched to ISO 6400 equivalent and see what happens. I am not expecting to see sparse histogram with spikes on an ISO 6400 native image.