A good variable ND filter is an ideal tool for videographers, filmmakers, and photographers. It gives you incredible flexibility for controlling the light that enters a lens. Variable ND filters can be adjusted to different levels of opacity, meaning you can control the light reduction as you like, and no need to change the filters as fixed ND filters.
A variable ND filter allows photographers to slow down the shutter speed to create amazing long exposure effects. These include creating milky seas and skies, erasing moving people from an urban scene and creating dramatic light trails.
These effect can be done with neutral density filters too, and will give you better pictures quality. However, these filters aren't adjustable. If you've got a 6-stop filter, and you want 8 stops of light reduction, you've no choice but to change filters. When you go out to take pictures, you have to take many ND filters. Yet you have to take time to change the filters.
1: A variable ND filter may present its density range as ‘ND8-ND2000’. At ND8 you'll get a 3-stop reduction in light transfer, and at the ND2000 position, there'll be an 11-stop reduction.
2: At maximum density you’ll see a cross effect that often creates an uneven exposure. So don't use the maximum density.
3: Variable ND filters can create unwanted effects with ultra-wide lenses, so longer focal lengths are best. If you use a ultra-wide lenses, it is better to select the lower NDs.
4: Don't choose the non-coating filters. They make your pictures very bad. The coating filters can help improve light transmission and reduce reflections.
5.Suggested shutter speed for different ND values.