Sony a9 III read noise vs ISO

The Sony a9 III (ILCE-9M3) was measured at 27 ISO settings from ISO 125 to ISO 51200. Values are Bill Claff's (Photons to Photos) direct measurements from raw files.

At base ISO (250) read noise is 2.73 DN, or 6.4 electrons at the sensor. A second gain (dual gain) kicks in at ISO 800, cutting input-referred read noise by about 1.3 stop. From ISO 800 up the camera is effectively ISO-invariant: raising ISO further doesn't meaningfully lower shadow noise. Settings above ISO 25600 don't change analog gain (digital push). Full well at base ISO is about 37,313 electrons; unity gain is around ISO 590.