Nikon D5100 read noise vs ISO

The Nikon D5100 was measured at 23 ISO settings from ISO 100 to ISO 25600. Values are Bill Claff's (Photons to Photos) direct measurements from raw files.

At base ISO (100) read noise is 1.28 DN, or 3.4 electrons at the sensor. From ISO 320 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 43,464 electrons; unity gain is around ISO 270.