Pentax 645Z read noise vs ISO

The Pentax 645Z was measured at 34 ISO settings from ISO 100 to ISO 204800. Values are Bill Claff's (Photons to Photos) direct measurements from raw files.

At base ISO (100) read noise is 1.19 DN, or 3.5 electrons at the sensor. From ISO 640 up the camera is effectively ISO-invariant: raising ISO further doesn't meaningfully lower shadow noise. Settings above ISO 2500 don't change analog gain (digital push). Full well at base ISO is about 48,430 electrons; unity gain is around ISO 290.