Dark noise subtraction.
Dark noise subtraction or dark frame noise subtraction is a method for reducing digital noise in long exposures. First, a photograph is taken with the shutter closed for the duration of the real exposure. This gives a map, as it were, of noisy pixels. Then another photograph is taken with the shutter actually open. The noise in the photograph can now be reduced by subtracting any noise present in the dark frame. Since both frames are taken under very similar conditions within moments of each other this is a reasonably successful technique of minimizing, but not entirely eliminating, noise. Dark noise subtraction can thus reduce noise from long exposures - typically night-time photography - at the cost of doubling exposure time and thus halving battery life.
Some digital cameras can use this technique automatically to reduce digital noise in exposures longer than a second or two. If your camera lacks this feature you can do it yourself manually using image editing software. A lot of amateur astronomers apply dark noise subtraction techniques manually, using image editing software, in order to achieve surprisingly good photos of the night sky from inexpensive consumer cameras. A variant of the technique is to subtract an averaged group of dark frame photographs.
cf. astrophotography, digital camera, interference, ISO, noise.
Entry last updated 2002-04-05. Term 310 of 1487.
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