Read noise is an important characteristic of a sensor and is often obtained simply by taking an exposure with the lens cap on and checking the standard deviation, which is the read noise since no photon noise is present. However, this method does not work with the D800e and other Nikon cameras which clip the read noise at zero, losing about a half of the normal distribution.
The D800e and many Nikon cameras have a masked pixel area where optical black has been applied so that no light reaches that part of the sensor. On the D800e this to to the extreme right of the frame and can be shown with RawDigger with zoom at maximum setting as shown below. The masked pixels start at column 7378 and extend to the right. The first 14 pixels are masked off and receive no light and can be used to get the read noise. The next two column are at saturation and the remaining pixels to the right are all at 1. This pixels are presumably for some type of calibration.
One can select the dark frame pixels using RawDigger's select by the numbers. I selected a 14 x 400 area as shown, and looked at the histogram. It is roughly bell shaped, indicating that the read noise is not clipped.
One then converts the selection to a sample and looks at the data. The standard deviations are the read noise.
One would think that the read noise would be constant, but when I looked at several frames taken at different exposures at the base ISO of 100, I noted that the read noise varies between 1.03 and 1.57 ADUs as shown in the table below, which is for the green channels only for simplicity.
Bill Claff also determines read noise by looking at the masked pixels with his proprietary program and obtains a 14 bit read noise of 1.261 ADUs for the base ISO of 100, somewhere in the midrange of my data. I am perplexed by why the read noise varies with exposure--does the camera do some type of preconditioning?