Neutral density filter

Neutral density filters reduce the amount of light entering the lens without altering it in any other way. It sounds an odd sort of thing to do, but ND filters are useful if you want to use slow shutter speeds for creative blur effects. In poor light, this isn't a problem because the shutter speeds will be lower anyway. But in bright light you need an ND filter to reduce the intensity.

You can also find 'neutral density' filters in some compact cameras, where the lens aperture adjustment may not be enough on its own to cope with really bright light. This is because, for technical reasons, the small lenses in compact cameras don't offer the small lens apertures you get with digital SLR and compact system camera lenses. In these cases, the neutral density filter is activated automatically.