There’s enough data in the test exposure that I talked about at the beginning of this series to calculate the full-well capacity of the photosites in each camera, if we make the reasonable assumption that the manufacturers of the cameras set the gain of the amplifier at base ISO so that the ADC clips at the moment that the well filled.
If the number of bits in the ADC is Badc, the base ISO is ISObase, and the Unity Gain ISO is ISOug
Full Well Capacity = (2^Badc)*(ISOug/ISObase)
Graphing the results versus the area of the photosites, assuming 100% fill factor:
Full-well capacity should be proportional to area, all else being equal, and for all but the CCD-based M9, it appears to be. This graph provides an explanation for the sketchy low-light performance of the M9.