purchased the K&F Concept Variable Neutral Density ND2-ND400 (9 stops) Variable ND Filter with Rotating Ring to be able to use a fast ND with different densities. The packaging is very well made, solid and compact container. The glass filter is light and the ring to increase/decrease the density is fluid. It does not have an end-of-travel lock. To test it at its best I tried it with three different lenses on a Nikon D750 camera (full frame): 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 that is easily resolved in post-production. As for the X-shape problem, it is quite marked using the filter beyond the marked ring. Using it instead inside the marked marking the problem is not very present. See attached image. - 35mm: in the photos taken with the filter there is no particular loss of sharpness, it introduces a slight warm dominant that is easily resolved in post-production. As for the X-shape problem, it is not as marked as in the previous one, but there is a vignetting especially when used at maximum effect. See attached image. - 85mm: in the photos taken with the filter there is no particular loss of sharpness, it introduces a slight warm dominant that is easily resolved in post-production. As for the X-shape and vignetting problem, they are not significantly present. See attached image. Optically, therefore, the filter appears to be well made and provides excellent results, but I would have preferred a start and end stop with a lock in order to avoid possible errors. Also considering that K&F produces other types of filters but with a magnetic attachment, I think it could be an additional plus for this filter. The quality/price ratio is correct based on the performance provided. I attach additional photos of the packaging and two shots at the seaside.