Nikon D1 read noise vs ISO

The Nikon D1 was measured at 6 ISO settings from ISO 200 to ISO 6400. Values are Bill Claff's (Photons to Photos) direct measurements from raw files.

At base ISO (200) read noise is 5.43 DN. From ISO 400 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).