The longer the exposure to record a scene, the more important noise reduction becomes. When you shoot nighttime shots, what should be a jet-black sky may exhibit tiny colored specks— artifacts—that put a considerable damper on your photo’s impact. (The longer the shutter stays open, the more artifacts you’ll get, as the camera’s sensor gradually heats up.)
A noise reduction feature usually works like this: When you press the shutter, the camera takes two shots—the one that you think you’re getting, and a second shot with the shutter completely closed. Since the camera’s electronics produce the visual noise, both shots theoretically contain the same colored speckles in the same spots. The camera compares the two shots, concludes that all of the colored specks it finds in the closed-shutter shot must be unwanted, and deletes them from the real shot.
Chances are you’ll have to dig through the literature or the specs on the manufacturer’s Web site to find out whether the camera you’ve got or are considering has this feature. But it’s worth investigating if you’re a nighttime shooter.
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