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Thereof, what is an ND filter used for?

The purpose of a standard photographic neutral-density filter is to reduce the amount of light entering the lens. Doing so allows the photographer to select combinations of aperture, exposure time and sensor sensitivity that would otherwise produce overexposed pictures.

which ND filter is most useful? 6-stop Neutral Density If you're a landscape photographer who wants to shoot long exposure sunsets, sunrises, waterfalls and moving water, the 6-stop ND is without question the best performing and the most versatile ND filter.

Also Know, do I really need ND filters?

You want to use a wide aperture so that you can create a shallow depth of field. However, with the bright ambient light you are constantly overexposing the scene, even after using a fast shutter speed. In a situation like this a neutral density filter works as a sunglass for your camera.

What is the difference between ND filter and polarizing filter?

Polarizers or polarizing filters work in a similar way, but the difference is that they selectively let light waves of a certain polarization pass through. This effect helps create more vivid colors in an image, as well as manage glare and reflections from water surfaces.