I purchased the K&F Concept Variable Neutral Density ND Filter ND2-ND400 (9 stops) with Rotating Ring to be able to use a fast ND with different densities.The packaging is very well made, solid container and not bulky. The glass filter feels light and the ring to increase/decrease density is smooth. It has no end stop block.To test it at its best, I tried it with three different lenses on Nikon D750 (full frame) camera : 20mm, 35mm and 85mm.- 20mm: in the photos taken with the filter there is no particular loss of sharpness, it introduces a slight warm dominant easily solved in post-production. As for the X-shape problem, it occurs quite marked using the filter beyond the marked ring. On the other hand, using it within the marked marking the problem is little present. See attached image.- 35mm: in photos taken with the filter there is no particular loss of sharpness, it introduces a slight warm dominant easily solved in post-production. As for the X-shape problem, it does not occur as pronounced as in the previous one, but vignetting is present especially when using it to maximum effect. See attached image.- 85mm: in photos taken with the filter there is no particular loss of sharpness, it introduces a slight warm dominant easily resolved in post-production. As for the X-shape and vignetting problem, they do not occur to any great extent. See attached image.Optically, therefore, the filter is of good workmanship and provides excellent results, I would have preferred, however, a start and end with blocking so as to avoid possible errors. Considering also that K&F produces filters of other type but with magnetic attachment I think it could be an additional plus for this filter.The quality/price ratio is correct according to the performance provided.I enclose additional photos of the packaging and two shots at the sea.