I've been reading and watching YouTube lately about dual gain sensors. For example the grain on a7IV (a7cII) sensor rises until ISO 400 where it resets to almost ISO 100 level. It starts rising again after the that. A1 sensor resets at 500 ISO and a7III (a7c) sensor resets at 640 ISO.
Then there are different base ISO for different PP.
Are those two related and how can we figure out what combination of PP/ISO will give the cleanest low light photos ?
From what I gathered sensor dual gain is hardware based, using dual circuitry to process the raw signal from the sensor. The second circuit activates at different ISO depending on the sensor, making it different on different cameras.
The dual ISO based on PP is software based, applying different noise reduction depending on ISO.
How sensor based dual gain and PP based dual ISO interact is quite a mystery !
Here is a list of lowlight gammas: Cine1, Cine2 & HLG1. These have been purposefully made for lowlight and have a higher noise reduction in shadows. You can also use Still Gamma. Because of the high contrast it will crush your blacks/shadows, and together with it, the noise.
Some people say you can find the dual base ISO by multiplying the base to x20
So if for cine1 base ISO is 100, dual base will be 2000, but I am not sure if that is true in all cameras. Haven't found any official papers confirming this idea.
At the moment, it doesn't seem to be a standard way of measuring the real base ISO, as different cameras have different values.
a7IV
S-Log3 - 800, 3200
S-Cinetone - 125, 500
No Picture Profile - 100, 400
a7SIII
S-Log3 - 640, 12800
S-Cinetone - 100, 2000
No Picture Profile - 80, 1600
By this logic, the a7IV in S-Log3 should have a higher second base, but it doesn't.
Sources https://www.keithknittel.com/articles/sony-a7s-iii-base-iso-for-s-log3-s-cinetone-amp-no-picture-profile
So overall, I cannot tell you the second base iso for all profiles as for me as well, it's unknown information.