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 that is already really good.

Now I was allowed to test the variable ND filter from the PRO series of K&F, a filter for no less than 280 euros. Of course, if you already work with filters, you should also use high-quality glass on a high-quality lens, but a filter for just under 280 euros is not a pittance and this investment should be well considered. What I can say in any case: The filter is not only from the price level but also from the performance and workmanship at the top.

It starts with the presentation (packaging) of this filter, neatly processed bag with magnetic lid, suede applications, soft and lint-free lined inside, a lateral belt with which the filter is transported upwards by pulling.
The filter itself is extremely well made and with the two brass lugs it is easy to use.
It has a very nice resistance when adjusting, not too heavy and not too light.
The glass shows no flaws and the threads are also absolutely clean cut.
About the image quality / image performance I can only say: TOP!
There are no color errors/color cast or anything else negative here. So I can work with my lens in bright sunshine even with open aperture and if I like also with a slightly closed aperture with the help of the filter realize long exposures.
Image quality is as I said with me as good as if I had no filter at all on the lens and so it must be.
The pictures I uploaded (undemanding test shots) were taken with the aperture open and against the sun.

My conclusion on this filter:
Very high quality workmanship/image quality flawless/haptics excellent. Price very high
Now everyone must decide for themselves whether such an investment comes into question.