Rather than use dark noise as a stimulus as in the preceding post, I have made a series of measurements of the histograms of the Nikon D4 and D800E, and the Sony RX-1 and NEX-7 when presented with a featureless surface eight or nine stops below clipping. I did not include the Leica M9 because I expect that it will perform pretty much like the D4, except with a lot more noise; if anyone would like to see that camera tested, let me know and Iíll oblige.
Each camera was measured with the camera ISO setting at all the whole stops from ISO 100 through ISO 6400. Shutter speeds were kept at 1/60 and faster to avoid any in-camera processing that might take place at slow shutter speeds. The central 200x200 pixels (10,000 pixels in each color plane) were used for the histogram. These tests have demonstrated to me that the histogram combing observed at high ISO settings in the previous post are due to two reasons:
ADCs that, although they are specified as 14-bit devices, are not delivering 14 bits of resolution
Digital gain applied by the camera manufacturers to the real raw data before it is written to the raw file.
The digital gain seems to be applied at very high ISOs, where there is so much noise that the l0ss in resolution of taking a 14-bit unsigned integer and multiplying it by a number less than 16 to yield another 14-bit unsigned integer probably does not adversely impact image quality.
One thing that surprised me is the Gaussian look to the noise in all cases. I had thought that the noise had longer tails than that from my dark noise tests.
Summary for the four cameras:
The Nikon D4 uses all 14 of its bits all the time except for those lost to digital white balance. There is no evidence of histogram periodicities that would indicate the elusive electron quanta, but with a sample that big, we've probably got several ADCs and several analog amplifiers involved. Details here
The Nikon D800E is at 14-bit device with all codes present except for those lost to digital white balance until the ISO knob gets to 3200. Then the gain of the analog amplifier stops increasing and the output of the ADC is shifter one bit to the left, giving thirteen bits of resolution. There is another one bit leftward shift and concomitant loss of resolution to 12 bits at ISO 6400. Details here
The Sony RX-1 is never a 14-bit instrument. It starts at 13 bits at ISO 100, and loses another bit in two of the channels at ISO 6400. Details here
The Sony NEX-7 starts out a 12-bit camera at ISO 100, is 11 bits at ISO 3200, and 10 bits at ISO 6400. Details here