Thanks Eric
I actually just did some more test shots this time with the lens cap on and a fast shutter speed to try and just record read noise. I'm currently analysing those with RawDigger but it might also be worth doing something like you suggest to get a better visual of the noise. I'll provide an update when I can.
Dave
Here are the results of my latest tests. All three shots were taken with the lens cap on in a dark room with a fast shutter speed. The details of the three shots are
1. ISO 100, 14 bit capture
2. ISO 100, 12 bit capture
3. ISO 800, 14 bit capture
All shots have had exposure increased by 7 stops in pp as well as 100% shadow increase, purely so the noise is visible.
I did an analysis of the sample distributions from RawDigger to estimate Standard Deviation of the noise and hence Dynamic Range. Unlike Canon cameras, Nikon have no offset voltage applied to the black level. This means that negative values of noise voltage are all read as 0. I tried to adjust for this by assuming that the distribution of negative values was the same as the positive values and substituted some of the 0 values with these negative values.
The noise calculation for shot 1 (ISO 100) gave an estimated DR of just over 14 stops, which correlates well with the DXOMark results for this camera. The calcs for shot 3 (ISO 800) also gave good correlation around the 12 stop figure.
With shot 2 (ISO 100 with 12 bit), the only sample values were 0 and 1. This is because of the reduced bit depth which can't resolve this low noise figure reliably. This also shows up in the appearance of the shot.
However given the appearance of the images, these results indicate to me that changing from 14 bit to 12 bit for a camera with DR of around 14 doesn't really change the level of noise to any great extent (and hence there is little change to the DR based on the definition of DR= Sat level/Read Noise level.
I'd welcome further comments please.
Dave