Beautiful blurred water
Neutral Density filters lessen the flow of light into your lens, enabling you to lower your shutter speed (making a tripod a must) for a blurred water effect. ND filters come in different strengths or ‘stops’, from subtle to almost opaque. The strength you need will depend on how slow you want to go. Each stop lets you double your shutter speed, so a three-stop ND reduces 1/60 sec to around 1/8 sec, while a 10-stopper takes 1/60 sec to 15 seconds.