Sony a7S II read noise vs ISO

The Sony a7S II (ILCE-7SM2) was measured at 40 ISO settings from ISO 50 to ISO 409600. Values are Bill Claff's (Photons to Photos) direct measurements from raw files.

At base ISO (100) read noise is 0.99 DN, or 10.1 electrons at the sensor. A second gain (dual gain) kicks in at ISO 2000, cutting input-referred read noise by about 1.7 stop. From ISO 40000 up the camera is effectively ISO-invariant: raising ISO further doesn't meaningfully lower shadow noise. Settings above ISO 80000 don't change analog gain (digital push). Full well at base ISO is about 160,685 electrons; unity gain is around ISO 1010.