Pentax K-3 III Monochrome read noise vs ISO

The Pentax K-3 III Monochrome was measured at 40 ISO settings from ISO 200 to ISO 1638400. Values are Bill Claff's (Photons to Photos) direct measurements from raw files.

At base ISO (200) read noise is 1.96 DN, or 3.5 electrons at the sensor. A second gain (dual gain) kicks in at ISO 800, cutting input-referred read noise by about 1.1 stop. From ISO 800 up the camera is effectively ISO-invariant: raising ISO further doesn't meaningfully lower shadow noise. Settings above ISO 6400 don't change analog gain (digital push). Full well at base ISO is about 29,017 electrons; unity gain is around ISO 360.