One way to determine the camera gain is to plot the variance of subtracted pairs of duplicate frames (the subtraction is done to remove fixed pattern noise, leaving shot noise and read noise) against the data number, as outlined by Christian Buel.
Yes, that's the common method of determining the actual situation for one's specific sensor array.
The gain is equal to the reciprocal of the slope ...
Correct, with the small precaution that the lower exposure pairs results will have slightly elevated noise levels due to the proportion of read noise added to the photon shot noise. Also, one should try and avoid any clipping. During exposure of the highest exposure pair watch out for clipping of the upper tail of the Poisson noise distribution (your data may have incurred some tail clipping), and when subtracting the lower exposure pairs it can be necessary to add an offset to avoid results below zero being truncated. When all these precautions are applied, then a reasonably good linear regression fit should be possible.
and the read noise is equal to the square root of the intercept.
I prefer to use a pair of 1/8000 second exposures (least possible amount of thermal noise (dark count) accumulation), with the bodycap on (instead of a lens with potential electronic interference and light leakage), and the viewfinder blocked (to avoid stray light from entering the mirror box). That will produce a slightly more accurate read noise only value than the intercept of a linear regression which may be influenced by some of the other noise averages. Some sensors have light shielded areas which can be used as no exposure areas, but I would even then try to use the shortest possible exposure time to reduce any thermal accumulation effects.
I obtained data for the D800e as shown below. The data are for ISO 100.
The slope is 0.2287, so the gain is 1/0.2287 or 3.46 e-/ADU. The read noise is sqrt(17.026) or 4.13 e-. The previously determined read noise was on average 1.05 ADU, so the read noise in electrons is 1.05*3.46 = 3.64 e-. Since the sensor saturates at 15735 ADU, the full well would be 15735*3.46 or 54503 e-.
That's an amazing capacity for such small sensel areas (~2289 photons per square micron), great technology. Together with the low noise floor that really helps the DR.
The Sensogren data for the D800 at base ISO gives a read noise of 2.7 e- and a full well of 44972 e-.
They're both lower than your observed values, maybe due to their DxO derived nature.
The engineering DR would be 54503/3.64 or 13.7 f/stops, which is in agreement with the DXO value of 13.24 stops.
My tests also show that the DxO conclusions (for 'screen' DR) in general are close to my own observations (like the determination I made for my 1Ds3 in a similar fashon for gain
and Read noise
as you did).
As a check, Bill Claff's data for the D800e lists the read noise at ISO 100 as 4.167, in essential agreement with my value of 4.13. I conclude that the Sensogren data for the D800 at base ISO is not accurate. They derive their value from reverse engineering of the DXO data and their methods have been questioned. Comments are welcome.
In general, I prefer well executed empirical data experiments over derived/converted data adopted from other sources.