Pentax K-3 read noise vs ISO

The Pentax K-3 was measured at 28 ISO settings from ISO 100 to ISO 51200. Values are Bill Claff's (Photons to Photos) direct measurements from raw files.

At base ISO (100) read noise is 1.92 DN, or 2.9 electrons at the sensor. From ISO 1600 up the camera is effectively ISO-invariant: raising ISO further doesn't meaningfully lower shadow noise. Settings above ISO 20000 don't change analog gain (digital push). Full well at base ISO is about 24,761 electrons; unity gain is around ISO 150.