Cinematographers like Shane Hurlbut, ASC, have stuck to using traditional ND filters over variable NDs because using two stacked polarizing filters isn't ideal for most motion capture. When shooting motion with a polarizer in front of the lens, the polarized light changes angle, which will sometimes alter your colors (skies, reflections, etc.), especially at higher densities.
...A variable ND should be sufficient, especially if your work will end up on an HD screen or the web.But for most indie shooters, a variable ND should be sufficient, especially if your work will end up on an HD screen or the web. One of the best things about shooting with a variable ND filter is that you'll only need one filter for each lens, which saves time on the set. If you're shooting without a matte box, you'll need several ND filters for each lens, which not only is expensive, but makes it difficult to keep track of if you're shooting in a run-and-gun style.
Until recently, your only option for variable ND filters were the Vari-NDs from Singh-Ray (www.singh-ray.com). Now, Tiffen (www.tiffen.com) has released the Variable ND, which incorporates seven ND filters into one. Made with high-quality optical glass, the Variable ND uses Tiffen's established ColorCore™ technology, offering far better optical performance. The filter material is laminated between two pieces of optical glass that's ground flat to a tolerance of .0001 inches and mounted in a precision aluminum ring. The Tiffen Variable ND filter gives you two to eight stops of light control and can rotate to whatever density is needed. It also has wider outer optics that help reduce the vignetting you typically get with wide-angle lenses. Although the Variable ND is only available for the 77mm at the moment, Tiffen is releasing new sizes in the near future, including a 52mm, 58mm, 62mm, 67mm, 72mm and 82mm. The 77mm retails for $239.99.
Schneider Optics (www.schneideroptics.com) has released a 77mm True-Match Vari-ND Thread filter that produces a density range from 11⁄3 to 11 stops of light loss. The True-Match also gives off no blue color shift at maximum density—a common problem with most variable ND filters. Like the Tiffen Variable ND, the Schneider only comes in 77mm, but they offer numerous step rings to adapt the 77mm size to smaller lens-thread diameters. The 77mm True-Match Vari-ND Thread retails for $495.
Although traditional ND filters are still recommended for professional work, a good variable ND filter can do the job for most applications. Keep in mind that the single filters are way more cost efficient and simplify the filtering process when shooting out in the field.