Pentax K-1 II read noise vs ISO

The Pentax K-1 II was measured at 40 ISO settings from ISO 100 to ISO 819200. Values are Bill Claff's (Photons to Photos) direct measurements from raw files.

At base ISO (100) read noise is 1.11 DN, or 3.3 electrons at the sensor. From ISO 160000 up the camera is effectively ISO-invariant: raising ISO further doesn't meaningfully lower shadow noise. Settings above ISO 204800 don't change analog gain (digital push). Full well at base ISO is about 48,634 electrons; unity gain is around ISO 300.