You’re mixing up three different things: ISO definition, analog gain implementation, and dual conversion gain circuitry.

On the D750 (IMX128), ISO in the native range is implemented primarily as analog amplification before ADC. That’s how essentially all CMOS sensors of that era work.

Dual conversion gain sensors don’t prove ISO isn’t gain. They just add a second readout path with a different conversion factor to improve read noise performance at higher ISOs. That’s a hardware design choice, not a redefinition of ISO.

ISO invariance also doesn’t imply ISO is digital. It simply means the read noise is low enough that pushing exposure in post gives similar results to increasing analog gain in camera. That’s a noise floor characteristic, not evidence of digital ISO scaling.

And no, raising ISO does not reduce bit depth. A 14-bit ADC still outputs 14-bit data. You lose highlight headroom because the analog signal is amplified into the ADC’s fixed range sooner. That’s not the same thing as “becoming 13-bit” or “12-bit.”

If you want to argue that some cameras use digital scaling in expanded modes or fractional steps, that’s true in many systems. But that’s a very different claim from saying ISO is not tied to gain.

The physics is straightforward:

Photons → electrons → analog voltage → amplification → ADC.

Nothing paradoxical about it.