Which ND filter?
Neutral Density filters enable you to block the flow of light into your camera, thereby extending your shutter speed. The strength of filter you need will depend on the ambient light on location and your exposure choices. Here we used a set of Firecrest filters from Formatt-Hitech that included six, 10, 13 and a whopping 16-stopper. Every stop enables you to double the exposure length, so consider that if you meter 1/125 sec without any filter, attaching a six-stopper will slow this down to 1/2 sec, while a 16-stopper will take it longer, to 8 minutes and 44 seconds.