Canon EOS 1D X Mark III (ES) read noise vs ISO

The Canon EOS 1D X Mark III (ES) was measured at 35 ISO settings from ISO 50 to ISO 819200. Values are Bill Claff's (Photons to Photos) direct measurements from raw files.

At base ISO (100) read noise is 3.51 DN, or 22.2 electrons at the sensor. From ISO 6400 up the camera is effectively ISO-invariant: raising ISO further doesn't meaningfully lower shadow noise. Settings above ISO 102400 don't change analog gain (digital push). Full well at base ISO is about 100,477 electrons; unity gain is around ISO 630.