The Kenko NDX filter can be adjusted from ND2.5 to ND1000. The filter can
darken the scene by anywhere from 1.3 to 10 EV. The filter allows the use of
wide apertures while keeping the correct exposure within the shutter speed range
of your camera.
Kenco suggests using this filter for shooting video with shallow
depth-of-field, for instance. Its variable nature allows the amount of
filtration to be reduced for focusing, then increased to the desired amount,
without having to remove the filter. Please note that the filters can vignette
when used with lenses wider than 28mm equivalent and are available in 77mm and
82mm filter thread sizes.