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Dark-frame subtraction is a technique used in digital photography to minimize picture noise in photos taken with extended exposure durations, high ISO sensor sensitivity, or at high temperatures. It takes use of the fact that two types of picture noise are consistent from shot to shot: dark current and fixed-pattern noise. Hot pixels, which light up brighter than surrounding pixels, are part of the image sensor noise. The method works by capturing a picture with the shutter closed and then removing it from the original shot with the noise.

The left side of the image has been subjected to dark-frame subtraction. The image sensor is used for the right half.
An image captured with the sensor in complete darkness is known as a dark frame (i.e. with a closed shutter or the lens and viewfinder capped). A dark frame like this is simply a picture of sensor noise. To adjust for fixed-pattern noise, a dark frame, or an average of multiple dark frames, can be removed from succeeding pictures.

Because the brightness of fixed pixel noise is reliant on both, it’s critical that dark frames be shot at the same ISO sensitivity and exposure duration as the original photo. A single black frame can be removed from many shots taken at these settings in these conditions, resulting in time savings and noise reduction for stacked star trail images that do not allow interruption.

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

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