Canon EOS 40D read noise vs ISO

The Canon EOS 40D was measured at 14 ISO settings from ISO 100 to ISO 3200. Values are Bill Claff's (Photons to Photos) direct measurements from raw files.

At base ISO (100) read noise is 5.62 DN, or 16.1 electrons at the sensor. From ISO 1250 up the camera is effectively ISO-invariant: raising ISO further doesn't meaningfully lower shadow noise. Settings above ISO 1600 don't change analog gain (digital push). Full well at base ISO is about 43,786 electrons; unity gain is around ISO 280.