ND and polarising filters are often mentioned together, but they perform different tasks.
ND filters: less light
An ND filter reduces the amount of light entering the camera. This allows a suitable shutter speed to be maintained in bright daylight. At 25 frames per second, around 1/50 second is a useful starting point for video.
The filter does not automatically make water softer. It only gives more control over shutter speed, aperture and ISO. The appearance of the water still depends on its movement and the camera settings.
Polarisers: control reflections
A polarising filter can reduce reflections on water, wet stones and leaves. This may reveal detail below the water or bring out the colour of foliage.
Reflections can also be part of the atmosphere. Light on a stream or sky on a lake should not be removed automatically. Turn the filter slowly and look for the right balance rather than the strongest effect.
With a very wide view, a polariser may darken the sky unevenly. Stacking filters can also produce dark corners, reflections or colour casts.
Try it directly
- Use an ND filter when there is too much light for the chosen shutter speed.
- Compare the polariser with and without its effect.
- Recheck exposure, colour and edges after fitting a filter.
- Leave the filter off if it removes an important reflection or the natural quality of light.
In short
An ND filter controls light. A polariser controls certain reflections. Both are tools, not automatic improvements.