bought the K&F Concept Variable Neutral Density ND2-ND400 Variable ND Filter (9 stops) with Rotating Ring to be able to use a fast ND with different densities. The packaging is very well done, solid and space-saving container. The glass filter feels light and the density increase/decrease ring is smooth. It does not have an end stop. 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, no particular loss of sharpness is noticed. introduces a slight warm dominant that can be 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 within 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 can be 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 to 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 can be easily resolved in post-production. As for the problem X-shape and vignetting are not relevant. See attached image. Therefore, optically the filter is well made and provides excellent results, however I would have preferred a start and stop with block in order to avoid possible errors. Furthermore, considering that K&F produces filters of other types but with a magnetic attachment, I think it could be a further plus for this filter. The quality/price ratio is correct on the basis of the services provided. I am attaching further photos of the packaging and two shots at the sea.