Canon EOS 1D read noise vs ISO

The Canon EOS 1D was measured at 12 ISO settings from ISO 100 to ISO 3200. Values are Bill Claff's (Photons to Photos) direct measurements from raw files.

At base ISO (200) read noise is 2.10 DN, or 11.2 electrons at the sensor. From ISO 500 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 21,961 electrons; unity gain is around ISO 1070.