After doing some research on google, this is an issue with most variable ND's when pushing the limits of the ND filter, especially with wide focal lengths. I've never used one, but have thought about getting one. They seem too expensive for what they are since I can stack a circular and linear cheaper. Normally I use a 2 or 3 stop for waterfalls. On the rare occasion, I might stack them which works fine other than some vignetting at the wide angle. I've never had the need for more than 5 or 6 stops though.