Variable ND filters allow variable degrees of light blocking depending on the orientation of the filter. They are only available as a circular screw on filter therefore,Read This Article regarding the advantages and disadvantages of screw on vs square filters.
Variable ND filters have a significant advantage in terms of being able to quickly adjust the amount of light blocked by simply turning the adjustment knob. This makes it most useful for situations where you may need to adjust exposure for a given shutter speed quickly; a feature that is more useful for videographers rather than still landscape photographers.
Take care when using a variable ND filter at its darkest extreme as you may start to notice some cross-polarisation artefacts. The NiSi PRO Nano Enhance ND-VARIO has conquered this problem, enabling higher quality for videos and images.
by Dylan Toh
NiSi PRO Nano Enhance ND-VARIO Review by Cristi Kerekes