Actually, you don't need a UV filter anymore these days, because camera manufacturers have been installing such a filter in front of the sensor for many years. However, there is one application in which a magnetic filter is useful and practical: If you have already fitted your lens(es) with a magnetic filter adapter, then a lens hood will generally no longer fit. However, this offers the best protection against, for example, accidental bumping. Without a gel, the front lens is exposed. And that's where this UV filter comes in, but a neutral glass filter would have done just as well. Now, filters generally have a negative effect on image quality, the only question is how much. This filter is very cheap at around 20 euros (67 mm) and I think the quality is OK. Flares (backlight that leads to unwanted reflections within the lens) seem to be much stronger to me, but it depends a lot on the lens anyway and a lens hood is always better than a better filter, unless you can avoid flares or even use them deliberately. There are two aspects of handling that bother me: the filter box is designed for much larger filters (I estimate 82 mm) and does not have a suitable recess for this 67 mm filter. This means that not only is the box much larger than necessary, but the filter also rattles around in it during transport. Solution: use your own filter bag, in the right size. The magnetic holding force between the filter and the lens adapter is weaker than that between the filter and the front cover. If you take the front cover off, this UV filter is always attached to it. For 20 euros, this filter is extremely cheap, technically/optically OK, but in practice it does show some small weaknesses. The fact that you basically get protection that can be removed quickly and that you don't have to spend a long time screwing on makes the filter still worth recommending.

Nice filter. Tested on a Canon 24-105 L lens. Lens cap fits fine, but the hood is a no go-either before or after applying the filter. Easy to use, good damping on the adjustments, stay where you put them, threaded on as good or better than any filter I've used. Biggest issue for me is having this with me when I need it. I expect this was designed by a waterfall guy. Can’t wait to try it when the weather breaks and I can finally get out to one. I had the best experience using a CPL when I was at Yosemite. Don't go there without one. This combo will be just the ticket when I return.

K&F Concept Nano-X variable ND with thread vs. K&F Concept Nano-X Magnetic vs. Freewell variable ND with thread

Looking for a variable ND filter for wide angles up to 24 mm focal length (APS-C 16 mm) I have the three filters ordered from amazon.de. Since prices are constantly changing, I just want to name the order. The K&F Concept Nano-X ND with thread is the cheapest, the K&F Concept Nano-X Magnetic is the middle priced one and the Freewell with 77mm thread is the most expensive.

I took photos with the 3 filters on the brand new Nikkor Z 4 / 24-120 mm and the Fujinon 2.8 / 16-55 mm.

The darkening of the K&F Concept Nano-X ND and K&F Concept Nano-X Magnetic are identical. The Freewell starts with a slightly stronger darkening. About 1/4 stop or time.

The 3 filters differ significantly in terms of vignetting (dark image corners). The K&F Concept Nano-X Magnetic performs worst on the Nikkor Z 4 / 24-120 mm with a focal length of 24 mm (see photo). If it absolutely has to be magnetic, I would go for a larger filter diameter and use a reduction ring. The K&F Concept Nano-X ND vignettes a little less (not at all on the Fuji 2.8 / 16-55 mm). The variable ND filter from Freewell achieves the best vignetting results thanks to its very flat design.

I find the K&F Concept Nano-X ND best packaged for transport. Here you get a nice round box with snapper. I liked the square plastic box from Freewell the least, but you can buy a very good (artificial) leather box with a zipper for a total of 5 filters from Freewell.

I am happy if my contribution is considered helpful and makes the decision easier.

I was a bit skeptical after the reviews that said that the filter was 0.5 mm too large and that a lens hood on the outside of the lens no longer fit. This is NOT the case at least with the Nikkor lenses 20 mm and 24 - 120mm Z with 77mm filter thread. Everything fits perfectly. I also couldn't make out the supposedly protruding groove on the 77mm UV filter. Visually everything is as usual with K&F 1a.

Great, finally I can take wide-angle photos with my 14-24 mm, 2.8 G from Nikon. However, you have to be careful when focusing manually. I recommend a tripod.

I'm really happy that this filter can be screwed onto my Nikkor Z 24-70 f/4 S with the lens hood in place. Obviously I need to reach into the lens hood to adjust the filter, but that's ok. No vignetting that I can see.
The filter works great, nice smooth action and a great range of density.

Works very well, I only recommend buying an additional lens hood because if there is backlight in the picture, it looks purple in the picture. Nevertheless, I am very satisfied, it does what it is supposed to do, I can't complain

I'm really happy that this filter can be screwed onto my Nikkor Z 24-70 f/4 S with the lens hood in place. Obviously I need to reach into the lens hood to adjust the filter, but that's ok. No vignetting that I can see.The filter works great, nice smooth action and a great range of density.