Since I have not yet worked with ND filters, but I do not know what I could complain here.
Overall, I am very satisfied with the glasses from K&F. You get a high-quality, processed filter with a top optical quality and a good scope of delivery (leatherette case).

Good quality of the filter.

Too bad that the accessories to reuse the filters that we already have are so difficult to order.

I had never owned a K&F Concept filter before and was pleasantly surprised when I first used it in my landscape photography. The reverse graded ND filters of the popular brands of photo filters tend to be quite expensive and this one in particular is more affordable and achieves results that match and even better than the most reputable filter brands. It's made of glass, which is an advantage over resin filters as it doesn't scratch easily. I really liked the filter cover, which is made of leather with inner protection. In short, I really liked this filter and I am sure that I will use it very often in my landscape shots.

I've been using the product for about half a year now (almost daily) and I'm very happy with it! Pro: Installation is easy and holds up very well and with the installation rings it can also be used universally for any lens! The filter also holds its Promise and the quality is still excellent!

I am very satisfied with the optical quality of this filter. Especially in comparison (e.g. with Cokin) it proves that you don't work with a plastic toy. The transitions are smooth and not immediately (frighteningly) noticeable

I received the new X-PRO filter set today and have already tested its applicability in the apartment. I'll give you a verdict on the quality of the filter glasses and how they can be combined in one shot... The box supplied holds the holder + adapter rings + CPL + ND 8/64/1000. The compartments can be pulled out on the drawstring and the filters can then be removed very easily. The plastic filter frames are discreetly sharp-edged, but not uncomfortable, and allow the filter to be removed and handled without having to touch the glass. Thanks to the tabs on the frame, the filters can also be pulled out of the holder very easily, especially in any order. I ordered two empty frames in 100 X 150 from the manufacturer's website, which should hold my graduated filters, which I have from another manufacturer, upon receipt. I'm curious how well this works. If it works, then I have a complete system for myself, with CPL, three ND filters and two graduated filters (Soft 0.9 and Reverse Soft 0.9). Sliding the filters in and out of the holder requires some force, but isn't awkward or imprecise. In this way, the graduated filter will also be easy to align. The handling of the CPL filter with the insert bracket is great, the turning mechanism is a bit rough, but precise. Screwing the adapter rings onto the lens works very well, the threads are cleanly worked, so nothing rubs. Attaching the holder to the adapter ring is also a joy and the presence of a set screw is very welcome as the holder rotates on the ring with relative ease. The handling of the system has already convinced me, I can live well with "only" two slots for my needs and I'm looking forward to the first use, in the next few days, in the great outdoors and with cold fingers. So far I've worked with round screw filters and dispensed with gradient filters, which I would like to change now. I also looked at the system from Rollei (Mark II), which cannot keep up with the handling of the X-PRO from K+F. Everything is very flat and fiddly, offers hardly any contact surface for holding and assembling the parts. Getting the polarizing filter or adapter ring out of the holder is no fun and almost impossible with cold fingers. K&F solved this much better. If you want to use gradient filters, the X-PRO set is a good choice. If you only need gray filters, I would recommend a magnetic round filter set with a diameter that is significantly larger than your largest lens, which is even faster to use, is more compact and should then do not darken with several filters in parallel.

I have long played with the idea of buying a graduated filter, but the prices of Nisi, Haida, etc. around 150 euros have always deterred me. Then I came across the filters of K&F, since the filter was in a Preislich favorable frame I ordered them.

Finally found an ND filter which can be installed inside the Smallrig lightweight matte box 2660. I did have to remove the holder frame to make it fit. This can be done by carefully pulling appart the frame sides with the lips. In doing so you can op out the filter while keeping the frame intact. Filter quality seems to be ok and it comes with a very well designed holder with an extraction ribbon which avoids having to touch the surface of the filter when taking it out.

The quality is convincing. No blurring or color changes. There is a case to go with it. Price performance convinces me completely. Clear purchase recommendation.