Low-signal histograms at various ISOs — Nikon D800E

At ISO 100, the D800E shows all codes in use except for the red and blue channel WB digital gain dropouts:

At ISO 200 there are no surprises:

At ISO 400, just some more noise. You can see that the blue channel has more digital gain applied to it than the red channel:

At ISO 800, just more noise:

At ISO 1600 the red channel starts to clip. We start to see unoccupied buckets in both green channels. We are about five times the Unity Gain ISO, and the green channel dropouts have about that periodicity, but I’d be surprised to see counts fall that far in the presence of so much noise. There are also dropouts in the red and blue channels that don’t have the same period as the dropouts we saw at lower ISOs that I’ve ascribe to digital gain:

At ISO 3200, we lose one bit of resolution. My guess is that the Nikon engineers decided that the signals are so noisy at ISO 3200 that there’s no point in cranking up the analog gain, and they just left the gain where it was for ISO 1600 and shifted all the numbers one bit to the left:

At ISO 6400 we lose another bit of resolution. Now everything is shifted two bits to the left. Note that the size of the WB gaps in the red and blue channels are doubled, indicating that the WB digital gain is sooner in the signal processing chain than the ISO digital gain.