Long Exposure Noise Reduction
A camera function that records a second, unexposed frame of equal duration and subtracts its noise pattern from the image.
In depth
The dark frame contains the same hot pixels, amp glow and dark current as the real exposure but no scene, so subtracting it removes those artefacts almost perfectly. The cost is that every exposure takes twice as long, which is unacceptable in a star-trail sequence or any timed interval — and in those cases dark frames are shot separately instead.
Examples in practice
A four-minute night exposure takes eight minutes with the function on, but returns free of hot pixels and corner glow.