A: It varies by camera, but, generally speaking, if your camera offers a long-exposure noise-reduction setting (most DSLRs do—look in setup or custom menus), set that and start with long exposures at lower ISOs. If objectionable noise results, dial down the shutter speed, and up the ISO into the 800–1600 range. And don't forget noise-reducing software such as Nik Software's Dfine 2.0 ($80, street). This Adobe Photoshop plug-in does an excellent job of eliminating noise while retaining overall sharpness.
(From Tech Support July 2009)