Sigma fp L read noise vs ISO

The Sigma fp L was measured at 31 ISO settings from ISO 100 to ISO 102400. Values are Bill Claff's (Photons to Photos) direct measurements from raw files.

At base ISO (160) read noise is 1.80 DN, or 4.2 electrons at the sensor. A second gain (dual gain) kicks in at ISO 400, cutting input-referred read noise by about 1.2 stop. From ISO 1000 up the camera is effectively ISO-invariant: raising ISO further doesn't meaningfully lower shadow noise. Settings above ISO 25600 don't change analog gain (digital push). Full well at base ISO is about 35,844 electrons; unity gain is around ISO 370.