This is the first time I have used a variable neutral density filter and I tested it on two different lenses on two different bodies. The first was on a Canon RF 24-105 f/2.8 mounted on an R7. Having the variable density was great while taking video in an area where there was a lot of sun but some pretty deeply shadowed areas as well. The filter turns pretty easily, so it's not difficult to change the reduction in light entering the lens, making it pretty simple to keep the right exposure while maintaining a wide aperture.

The second setup I tried was a Canon RF 15-35 f/2.8 mounted on an R6 Mark II. I used this combo to take photos, and this is where I ran into the shadowing at wide apertures. I've attached three photos that show the shadowing - two at 15mm f/2.8 and one at 35mm f/2.8. The two at 15mm were taken with the variable ND set first to 2 and then at 32. You can see the shadow effect at 15mm. I went to the K&F website and their answer for this problem is to simply turn the filter until the shadows go away, which admittedly does work. However, this is subpar and the reason I gave the filter only 4 stars. When you loosen it a bit (for me it was about a third of a turn) to make the shadows go away, it makes it much more difficult to change the density of the filter since there isn't enough friction to keep the base in place while you turn the front.

Overall this filter is great if you know you're going to use it at more than 20mm or so, and especially if you're using it for a shoot where you're not moving around a lot. But if you need to shoot wider and want to keep a wide aperture, you probably want to stay away.

Finally - build quality is good and solid, the threads were smooth, and the glass is clear. I am not a professional, but I didn't notice a color change with the filter on. The only other drawback is that my lens hood doesn't work too well with the filter, and there's no good way to put a cap on the filter, so again - if you're going to be in a static place then this is great, but if you need to move around more, you might want to consider other solutions.