ISO 50 is basically ISO 100 with some behind the scenes exposure trickery and a digital exposure shift to compensate
you LOSE DR at ISO 50 because of that
but the power levels and gain at the sensor are exactly teh same as ISO 100
every current commercial (consumer, prosumer, or pro) DSLR on the market these days uses ISO 100 as base ISO
some have an Expanded 50, some have Expanded ISO's above their maximum native
from ISO 100 through the maximum native ISO, the number of electrons per digital unit drops by a factor of two for each stop of ISO increase
so, fewer and fewer electrons are used to generate single digital units
at some point you have a crossover, where one electron is converted into one digital unit
unity gain is usually somewhere between ISO 200 and ISO 800
it might be closer to a third stop setting
it is rarely ever an exact selectable ISO setting
I guess technically speaking, the per-pixel amplifiers are still putting out voltage
rather than DU...digital units only come into play at the ADC
so, assuming per pixel amplifiers in a CMOS sensor convert input voltage to output voltage
I figure the formula would be:
if input voltage is 100,000 and output voltage is 50,000, gain would be -0.6dB