The Variable ND is a specialty filter designed to allow quick and continuously variable adjustment of light attenuation between approximately 2-6 stops, depending on conditions. Variable ND filters are very popular with HDSLR filmmakers because they allow the cinematographer to quickly and precisely adjust exposure.
The Variable ND is the quickest way to make exposure adjustments for the time constrained independent filmmaker. This filter uses a rotating ring to adjust filter density. Variable NDs are technically not NDs at all, but a combination of 2 specially designed polarizers which create an ND-like effect, meaning the filter has a slight polarizing effect.