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dark-frame subtraction

Wikipedia explanation

A photograph that undergoes dark-frame subtraction can qualify for the “Guaranteed TCQ” label.

Like flashing, dark-frame subtraction does not involve the addition of “pictorial information” from another exposure, an addition that is prohibited by Q5.

To not be disqualified, however, the additional exposure in black-frame subtraction has to involve only complete darkness so that there is no “pictorial” information from the scene being recorded.

It has to be equivalent to what would be recorded when clicking the shutter (high-shutter speed) on a camera with an opaque lens cap on the lens (fully stopped down). In a black camera bag. In the bottom of a locked closet. At midnight.