Pandimonium: Wonders why the ISO 800 shot looks cleaner than the ISO 100. Also, why does Sony persist in getting crucial stuff wrong (like raw compression).
@Azimuth46There is a different conversion gain inside the pixel at ISO 100 and ISO 640; but at ISO 32000 there is signal processing (noise reduction) and that falsely looks like improved noise.
Yes, there are essentially two ISO-invariant ranges; one starts at ISO 100 and the other at ISO 640 (some signal processing kicks in at ISO 32000). See:http://photonstophotos.net/Charts/RN_e.htm#Sony%20ILCE-7RM2_14