In the Mavic, we use ND filters as a substitute for an adjustable aperture to control the amount of light striking the camera's sensor. Ideally to achieve a slightly soft 'cinematic' effect, we want the shutter speed of the camera to be roughly double the frame rate. If you manually set the ISO value to 100 (the finest 'grain') on a medium sunny day with Auto exposure on, you will probably see a shutter speed of around 400. Let's say you are shooting 4K video at 30 frames per second, then you want a shutter speed of 60 frames per second. Try your middle ND16 filter and notice the effect. Still to fast? The go to ND32. Too slow? Then ND8 will be your filter. Clear as mud?