What it is, is kind of a bummer . . . you have to open the shutter without energizing the CCD (which otherwise acts as a big dust magnet, and which with Nikons requires a separate AC power supply) and use methanol to swab the dust off the CCD. Some folks seem to freak out about having to do this on occasion (I've done it twice in 4,000 images), but I didn't find it to be a problem. The problem is minimized by not changing lenses, but of course that defeats the whole purpose!