Dark-frame subtraction

The technique works by taking a picture with the shutter closed and subtracting that electronically from the original photo exhibiting the noise.

Such a dark frame is essentially an image of noise produced by the sensor.

[1] It is important that dark frames are taken at the same ISO sensitivity and exposure time as the original photo, because the brightness of fixed pixel noise is dependent on both.

[1] Under these circumstances, a single dark frame can be subtracted from multiple photos taken at these parameters, amounting to a time saving and allowing noise reduction for stacked star trail images, which do not allow interruption.

Dark-frame subtraction is also used in digital photogrammetry, to improve the contrast of satellite and air photograms, and is considered a best practice, along with flat-field correction, for astrophotography.

A dark frame from a Nikon D300 with enhanced contrast to emphasise noise
Dark-frame subtraction has been applied to the left half of the image. The right half is directly from the image sensor.