When taking a long exposure, the chip will introduce โthermalโ noise. Its level is magnified by three things โ temperature, exposure time, and ISO. Dark frames are used to subtract this sensor noise from your image and mitigate โhot or coldโ pixels. (Some modern sensors automatically calculate dark levels and donโt need dark frames). Dark Frames also will calibrate the chip so all pixels give the same value when not exposed to light.
