To get the nice smooth water with film or digital you need a long exposure time. With view camera most lens/shutters have apertures as high as F68 some higher! So getting a nice long exposure is not a problem, but most digital PS and DSLR's are limited as the F stops max out between F16-F32, with ISO's mostly starting about 200 which limits the long exposures. ND filters reduce the light so you can take longer exposures and get the correct exposure. But you also need a good tripod.