The Kenko Variable NDX filter offers an effect similar to that of a standard neutral density filter, but created by using stacked polarizing filters to provide the equivalent of ND 2.5 to ND 1000 brightness. Interestingly, Kenko claims that while the Variable NDX is based on polarizing filters, it won't affect color balance, glare, or reflections when used.
A couple of benefits are claimed for the design, as compared to traditional neutral density filters. With no need to stack multiple filters, there's no risk of vignetting. It's also conceivably easier to frame a scene with the Variable NDX filter set to its lowest strength, then adjust the filter strength by rotating the outer filter ring, rather than trying to attach a new filter to the camera without changing the framing.
Pricing and availability for the Kenko Variable NDX filter hadn't been disclosed at press time.
Kenko's Variable NDX filter.
Diagram provided by Kenko Co. Ltd.