I already have a few filters at home, including the K&F Concept Nano-X ND Filter 77mm variable gray filter ND2-32 for 75 euros, which I use on my wide angle, for example in bright sunshine (for open-aperture shots) and for long exposures and it's really good.

Now I was able to test the variable ND filter from K&F's PRO series, a filter for a whopping 280 euros. Of course, if you're already working with filters, you should also use high-quality glass on a high-quality lens, but a filter for just under 280 euros isn't cheap and this investment should be carefully considered. What I can definitely say: The filter is at the top not only in terms of price but also in terms of performance and workmanship.

It starts with the presentation (packaging) of this filter, cleanly processed bag with a magnetic lid, suede applications, soft and lint-free lined inside, a band that leads out to the side with which the filter is pulled upwards.
The filter itself is extremely well made and the two brass lugs make it easy to use.
It has a very pleasant resistance when adjusting, not too heavy and not too light.
The glass shows no defects and the threads are also cut absolutely cleanly.
All I can say about the image quality/performance is: TOP!
There are no color errors/color casts or anything else negative here. So I can also work with my lens with an open aperture in bright sunshine and, if I want, I can also use the filter to take long exposures with the aperture slightly closed.
As I said, the image quality for me is as good as if I hadn't had a filter on the lens at all and that's how it has to be.
The shots I uploaded (undemanding test shots) were taken with the aperture open and against the sun.

My conclusion about this filter:
Very high-quality workmanship/image quality flawless/feel excellent. Price very high
Everyone must now decide for themselves whether such an investment is an option.