Canon EOS 20D read noise vs ISO

The Canon EOS 20D was measured at 6 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 2.08 DN, or 24.9 electrons at the sensor. Read noise keeps falling across the measured range, so the camera isn't fully ISO-invariant there. Settings above ISO 1600 don't change analog gain (digital push). Full well at base ISO is about 47,537 electrons; unity gain is around ISO 1200.