Waterfalls are great to shoot with the aid of ND filters. In this case, this image was shot using a 10 stop ND filter. To know more about ND filters, click here.
If one wants to shoot long exposures of over 30 seconds then it will be necessary to shoot in bulb mode. What bulb mode allows us to do is to increase the shutter time from 30 seconds to infinity and for this task, we need a steady tripod, an ND filter and a remote release shutter cable. With an ND filter, we will know how much light we are blocking and then it becomes easier to calculate the new shutter speed. For example when composing the initial frame, if the shutter is 1/4 second and aperture is set to f/11, on using a 10 stop ND filter will increase the shutter to 4 minutes 16 seconds. For this we will need a shutter remote.
Neutral density filters come in various strengths or optical densities from blocking 1 stop of light to blocking out 10 or more stops of light passing through the camera lens. Each ND filter is given a notation of 2, 4, 8, 16, 32 and so on. The purpose of using ND filter is to achieve motion blur when using slow shutter speeds and are primarily used for shooting landscapes.