A slow shutter speed is necessary to give running water a silky effect. This type of photography is best done in low light, or the camera may not be able to reach the desired shutter speed without overexposure. Otherwise, if conditions are too bright, a neutral density [ND] filter can be used, which will block out extra light. Other subjects suitable for slow shutter photography include car light trails, ocean waves, fast moving clouds, and public places (to make people completely disappear from a scene or render them as blurred, moving shapes).