I spent a long time buying a filter because on the one hand I didn't know if I needed something like this and on the other hand because I had already tested a 0815 filter with miserable results and the costs for such a part are not insignificant. The question now was whether I should get several filters or a variable filter. In view of the price, the question ultimately became irrelevant. The first tests were able to dispel concerns that the image quality suffers, as with the filter mentioned at the beginning. The images are slightly blurry, but really only marginally. The weakest position extends the exposure time by a little more than 2. In my experiment (Nikon D7200, Sigma 18-35, K&F variable ND filter 77mm ND2-ND400 with adapter ring) I set the aperture to 2 in sunshine. 8 raised to expose well at 1/8000 without a filter. With the filter screwed on in the "MIN" position, the shutter speed went up to 1/3200. I turned the filter from mark to mark and came up with the following shutter speeds calculated by the camera: 1/3200, 1/3200, 1/2500, 1/2500, 1/2000, 1/1250, 1/800, 1/400, 1/125, 1/20(X-Cross, but underexposed). As you can see, hardly anything happens at first and the markings also do not allow any ND value order to be determined, which according to the logic of the first point is 2, the second 4 and the third 8.... So it's time to try it out. In the "MAX" position (1/20) the images are unusable because of the X-Cross, but this is really only the case in the absolute max position, and between the penultimate with 1/125 and Max intermediate positions are also possible where I got useful up to 1/30. So a usable ND value of 266 is calculated. This value is now completely sufficient for filming, because who says that I can't set the shutter speed higher than 1/60, which was the rule of thumb in the early days of analogue filming. Enough theory, I think the filter is an absolutely good investment.
Unscrewing is completely problem-free and if there was a color cast, it was due to the camera (Nikon D7200).
I've uploaded a few pics:
1.) 100% topless bottom view with filter at 5th position (1/2000)
2.) MIN - position
3.) Last usable position
4.) Filter to with X-Cross where the camera with the exposure of my camera (Nikon D7200) failed and I had to raise 2 values ​​in post-processing to be able to recognize something.

Pretty happy with this filter. The magnetic aspect makes it a breeze to put on and take off. No cross-thread filters or fiddling. The CLP works well and does what it's supposed to do. Reduces glare and enhances blue skies. A CPL is one of those filters I never go without, especially for landscape photography. The filter is well made and finely finished.

Great piece for landscape photography. Variable ND without "shadow cross", clean in both areas. Polarizing filter goes great, and if it does get too bright you'll love the ND, stepless control. Used on FZ200.

Great Variable ND Filter for the price

Great piece for landscape photography. Variable ND without "shadow cross", clean in both areas. The polarizing filter works great, and if it gets too bright you'll love the ND, stepless control. Used on FZ200.

Great to have a variable and an ND filter in one. No color cast visible. Recommend.

This K&F Concept variable ND filter is to product that does not disappoint the photoamateur and based on the price paid, the top rating is fully earned. Needless to say, it is not comparable to high-end ND filters.
The only weakness, as pointed out in other reviews, is that it has no start and end point, so the dial spins endlessly. However, the problem does not exist for landscape shots where you have plenty of time to check the correct filter setting.
Vignetting appears only below 24mm, instead the cross only at the last stop in precarious lighting conditions. Colors are vibrant and not altered, color dominance is minor and for the perfectionists it can be resolved in raw in post production.
The following photos were taken with the highest stop, only the night one was taken with lower stops.

Qualities:
- very good materials
- very good finish
- perfect threading
- colors are vivid
- color dominance is minor
- top cost/performance ratio

Flaws:
- it does not have end stops
- vignette under 24mm
- cross only at the 'last stop with precarious light

Rated, based on price, 100/100

Great piece for landscape photography. Variable ND without "shadow cross", clean in both areas. The polarizing filter works great, and if it gets too bright you'll love the ND, stepless control. Used on FZ200.