Long-Exposure Noise
As touched on earlier, “dark” or “fixed-pattern” noise is caused by imperfections and irregularities in the sensor, including, for example, uneven boosting between the individual readout amplifiers accompanying each photosite on a CMOS sensor.
It increases approximately in proportion to the exposure time, and at one or several minutes can be very noticeable. It is also enhanced by heat-generated electrons adding to the photosites, so you can expect more noise in warm weather than in cold. Because this kind of noise is related to the sensor and its circuitry and not to the image, it lends itself to fairly efficient correction. The key, however, is to record the exact pattern of the noise at the time of shooting. Some DSLRs ...
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