Nikon D500 read noise vs ISO

The Nikon D500 was measured at 38 ISO settings from ISO 50 to ISO 1638400. Values are Bill Claff's (Photons to Photos) direct measurements from raw files.

At base ISO (100) read noise is 1.38 DN, or 4 electrons at the sensor. A second gain (dual gain) kicks in at ISO 400, cutting input-referred read noise by about 1.1 stop. From ISO 8000 up the camera is effectively ISO-invariant: raising ISO further doesn't meaningfully lower shadow noise. Settings above ISO 51200 don't change analog gain (digital push). Full well at base ISO is about 45,986 electrons; unity gain is around ISO 290.