I did read that graph Colin, I have done so many times, I just interpret it differently.
As far as I am aware, the photoreceptor operates in the same way each time regardless of the gain setting and the gain is applied in an output amplifier like the block diagram below, and that makes all the difference in the world.
https://www.edmundoptics.com/content...-ucsmva-lg.gif
With the gain applied in a post pixel readout output amp, the charge capacity (Well depth in electrons) of any given pixel remains the same. Changing the gain on the output amp will change what voltage is applied to the ADC for a given number of electrons in the pixel well (Which matches the manufacturers specifying gain in terms of electrons per ADU count) Reducing gain will make the ADC less discriminating (More electrons required per ADU count change. Reduced precision in encoding) but able to quantise a greater total number of electrons in the same number of bits output. If gain is raised, it will be more discriminating (1 electron per ADU change for instance instead of 4, more precision) but the total number of electrons able to be quantised using a given number of bits drops, the sensor can continue accumulating electrons but it makes no difference to the output.
As far as I have ever been taught, and can see on any useful block diagram I have found so far, the PIXEL well depth remains the same independent of gain, but the systems
effective well depth will change with gain unless the ADC bit count is high enough to quantise the whole range of voltages fromt he readout amp after gain is applied. With 64ke wells at 4 electrons per ADU, 14 bits can quantise the lot, but at 1 electron per ADU, 14 bits can only quantise a quarter of what the sensor could accumulate.
If someone can point me to info that says the number of electrons the pixel wells themselves can store changes (The number of electrons per detected photon must also change) rather than the number of electrons that the ADC can
quantise changing I will be happy to be proven wrong. If not, the pixel well depth in question in that graph is the effective well depth at which point the output is saturated for the system as a whole, not the depth of the actual pixel wells on the sensor.
To put it in equivalent terms to my other expensive hobby (Race cars) The speedo in my HR1 Skyline only read to 180KMH where the new one will read to 300 if I wanted, but the original didn't stop the car from doing 220 at Phillip Island, it just couldn't display that speed.