I'm interested in buying a high quality variable ND filter to use on my Nikon lenses with a Metabones speed booster. I know that the Heliopan is highly recommended but for some reason the size I would be using (77mm) does not have good reviews. I'm not sure if they have some quality control issues for that size. I also don't like the fact that they don't have front threads for that product. I came across the B+W 77mm XS-Pro Digital ND Vario MRC nano Filter and it has some excellent reviews on B&H photo. Has anyone had any experience with this item? Would it provide enough stops (goes down to 5 stops or 1.5 density) for my purpose as I would like to try my Nikon 24mm 1.4g lens wide open with the speed booster which I'm presuming will be extremely bright wide open? An other concern is if it would cause any vignetting problems with this wide angle lens (although I guess it wouldn't be considered wide angle anymore on the GH4) or if I would encounter the X artifact that some people report with other filters. Here's the link to the product at B&H photo. It looks like a fairly new product and I don't see it being sold anywhere else. Thanks for your input.