I suspect the SNR comparison depends on the camera model (which includes the physical sensor, any analogue gain, the ADC, any digital gain, and whatever post-processing is done. Especially any noise-reduction, obviously.
I don’t know exactly how my camera works. When I twiddle the ISO dial, what changes? Is there a change in analogue gain, or what?
This is somewhat frustrating, but I never fully understood how film worked, even just B&W, let alone colour. I think my D800 uses analogue gain for different ISO, up to the “extra” ISO called H1, H2 etc which are digital. But I could be wrong.
Personally, I’m not much bothered about what the theory says should happen. Instead, I look at what actually does happen. For example, an out-of-focus gray card, under office lighting (LED), constant exposure 1/800s f/2.8, at ISO 6400 and ISO 100:
set AREA=3500 2500 300 300
set CROP=-crop 300x300+3500+2500
%DCRAW% -v -A %AREA% -6 -o 1 -T -O 4398.tiff AGA_4398.NEF
%DCRAW% -v -A %AREA% -6 -o 1 -T -O 4402.tiff AGA_4402.NEF
%IMG7%magick 4398.tiff -strip %CROP% -auto-gamma +write isonse_4398.png -colorspace HCL -format "%%[fx:mean.g]\n%%[fx:standard_deviation.g]\n" info:
%IMG7%magick 4402.tiff -strip %CROP% -auto-gamma +write isonse_4402.png -colorspace HCL -format "%%[fx:mean.g]\n%%[fx:standard_deviation.g]\n" info:
The numbers are:
ISO 6400:
0.156747
0.0983761
ISO 100 (given post-proc gain):
0.170328
0.132537
The numbers show that post-proc gain has higher saturation (mean chroma), and a wider variation (standard deviation of chroma).
In real photography, I use dcraw’s noise reduction for high ISO, which improves those images.
So, if post-proc gain is noisier, do I avoid it? No. On the contrary, I generally set the camera to auto-expose with exposure compensation up to -2 stops. I deliberately underexpose to avoid clipping highlights, then correct with post-proc gain. For me, some extra noise is a price worth paying to avoid clipping.