Announced way back in September 2010, the Kenko Variable NDX filter offers an effect similar to that of a standard neutral density filter, but with a variable strength that's created by stacking two polarizing filters to provide brightness equivalent to anywhere from an ND 2.5 filter to that of an ND 1000 filter.
Although the Variable NDX is based on polarizing filters, Kenko stated at last September's launch that its usage won't affect color balance, glare, or reflections. The company notes a couple of benefits 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 said to be 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.
In Japan, the Kenko Variable NDX filter will ship from August 26th, just three weeks shy of a year after it was first announced. Kenko's website lists two variants for the Japanese market. The Variable NDX 77S fits a 72mm filter thread, and is priced at ¥55,000, approximately equivalent to US$700 ignoring exchange rate fluctuations, taxes and duties. The larger Variable NDX 82S accommodates an 82mm filter thread, and lists for ¥65,000, roughly equating to US$850. Information on pricing and availability in other markets wasn't available at press time.
Kenko's Variable NDX filter.
Diagram provided by Kenko Co. Ltd.