A variable ND filter is a neutral density filter constructed from two stacked linear polarizers, the rear element fixed and the front element rotatable. As the front polarizer turns relative to the rear, the amount of light passing through decreases continuously, providing a smooth range of density without the need to swap filters. A typical variable ND covers two to eight stops, with markings on the rim showing approximate density values rather than precise stops.
The major advantage is convenience, particularly for video work. When the sun moves behind clouds and the scene brightens by two stops, the operator can dial down the variable ND without breaking the shot, where a fixed ND would force a swap and a re-rack. This makes variable NDs essentially mandatory equipment for run-and-gun video and event coverage, where exposure changes happen smoothly through the take rather than between setups.
The downside of the design is the cross-pattern artifact, often visible as a dark X across the frame when the filter is rotated near its maximum density. Cheap variable NDs show this clearly at any setting past the middle of the range. Better implementations from Tiffen, Hoya, Breakthrough Photography, and Nisi use higher-quality polarizers and tighter manufacturing tolerances, suppressing the cross-pattern until very high densities. Some premium variable NDs add a hard stop just before the cross appears to keep users in the usable range.
Color shifts are the other characteristic problem. Stacked polarizers tend to introduce a green or magenta cast that varies with rotation and with focal length. White balance correction in post handles most of this for stills, but video color grading needs to account for shifts that happen mid-clip if the filter was dialed during the take. Some manufacturers explicitly engineer for neutrality and publish color-response charts, which is worth checking before buying.
Fixed ND filters remain the gold standard for stills work, especially long-exposure landscape photography where neutrality matters and the exposure does not need to change during a shot. A 6-stop or 10-stop fixed ND will produce cleaner long exposures than any variable ND at equivalent density. Variable NDs are best understood as a video-first tool that also serves stills photographers who prioritize speed over absolute optical purity.
Common pitfalls include rotating into the cross-pattern zone without noticing, mixing variable ND with a polarizer (the rear element of the variable ND already is a polarizer, so adding another behaves unpredictably), and using a variable ND on ultra-wide lenses where the cross-pattern and color shifts become more pronounced across the wider field of view. Slim, multi-coated variable NDs from reputable brands minimize all of these issues but never eliminate them entirely.