Sony a7R III read noise vs ISO

The Sony a7R III (ILCE-7RM3) was measured at 34 ISO settings from ISO 50 to ISO 102400. Values are Bill Claff's (Photons to Photos) direct measurements from raw files.

At base ISO (100) read noise is 1.27 DN, or 4.2 electrons at the sensor. A second gain (dual gain) kicks in at ISO 640, cutting input-referred read noise by about 1.4 stop. From ISO 2500 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 52,477 electrons; unity gain is around ISO 330.