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.

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

The filter works very well on my Sony Alpha 7 III with a Tamron 24mm F / 2.8. There is no cross formation or other unsightly effects. The workmanship feels good and comes in a practical plastic case. As always, the shipping was great with Amazon. I can definitely recommend the 67mm version of the filter. BUT, the filter may not take into account that well in other sizes.

I am very satisfied with the optical quality of this filter. Especially in comparison (e.g. with Cokin) stell

I have used numerous filters, from Cokin, to buying several for different sized lenses. And, I do a serious amount of light and strobe painting (do a you tube search, if you're curious). So, when I dropped my super expensive ND filter, I was crushed! Right up until I found this K&F Concept ND, which offers a "Variable Neutral Density". Imagine looking at purchasing an ND Filter for the Typical four hundred bucks (and, that is only for a single fixed ND number)? Oooor, grabbing this well constructed, high quality ND filter that you can adjust to your liking ON THE FLY! Still thinking...
Buy it Bonehead!

I have used numerous filters from Cokin, buying several for different size lenses. And I do a serious amount of light and strobe painting (do a YouTube search if you're curious). So when I dropped my super expensive ND filter I was devastated! Until I found this K&F Concept ND that offers a "Variable Neutral Density". Imagine buying an ND filter for the typical four hundred dollars (and that's only for a single fixed ND number)? Oooor, grab this well constructed, high quality ND filter that you can tune to your liking ON THE FLY! I still think...
Buy it Bonehead!

Everything perfect

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