Olympus E-1 read noise vs ISO

The Olympus E-1 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 2.25 DN, or 18 electrons at the sensor. From ISO 100 up the camera is effectively ISO-invariant: raising ISO further doesn't meaningfully lower shadow noise. Settings above ISO 800 don't change analog gain (digital push). Full well at base ISO is about 32,271 electrons; unity gain is around ISO 800.