K&F Concept Adjustable ND filter with a very good price-performance ratio in top quality
You have to try a bit, since there is no scale, but with a little practice you can take great pictures.
(Picture was taken with a 2sec exposure at max ND filter value)

I haven't been able to test it much yet but from what I've used it's a good ND filter, good photographs and good effects are achieved. Great value for money

Really a very good ND filter!
I tested it directly in Hamburg and in the forest and it's just perfect!
The picture is very sharp and just looks great!
I put my two favorite pictures up with Rein.
The resistance to make the filter lighter or darker is very good and pleasant.

Overall a very great product!

After I needed a new neutral density filter for my DSLR, I decided on this K&F Concept model and was not disappointed. I'm an amateur photographer. Pros may have different needs, but I wasn't disappointed.

K&F Concept easily adjustable ND filter. The display on the outside shows the area in which the ND filter is set.
The filter can be easily rotated onto the lens because it has a good grip on the outside and is also easy to disassemble.
With the ND filter, long exposures can be carried out very well in daylight.

I'm really super satisfied with the K&F Concept ND filter, it looks high quality, price-performance is great and I've been able to use it many times. Thanks to the scaling on the outside, the permeability can be adjusted very well.
Gladly again

Variable ND filters are not recommended very often. There's the argument that the darkening range is too small, that it's more universal with a square filter that you can put in front of any object using an adapter... And finally, there's the argument that they often have a slight color cast. Which cannot be dismissed out of hand with these. I noticed a very, very slight green tinge.
Variable ND filters were developed for filming, because the lighting conditions change frequently and can be adjusted very quickly by rotating them. The slight color cast can be quickly removed in image processing. I find these filters good for my purposes, because I can experiment with the different darkenings due to the variability, the filters are inexpensive. I bought them for my most important lenses. Not only are they good for long exposures, but also when it's very sunny. By using them, I can achieve longer exposure times. This also applies to portraits. These filters can be easily screwed into the thread and - what is really important - also unscrewed again. Anyway, I'm looking forward to the first long exposures with it.
I've also heard that the screw filters can cause vignetting. These are also easy to remove with image editing. Moreover, they are even "modern" at the moment.
The filters now accompany me in my first steps in the field of long exposures. If I feel more professional, maybe my equipment will become more perfect. Now, however, I look forward to working with them.

Filter and variable entry level, really low cost. Good for learning how to use it.
Colors shift slightly to yellow.
The dial spins smoothly, but it does not have end stops, so it spins at infinity, which is annoying, as you need to have visual feedback on the dial to check the massive position.
The K&F Concept filter is fairly smooth, except at the point near the maximum, where it starts to vignette a bit. Beyond the maximum the X appears, and it is therefore unusable.
In any case a good expense to practice with ND filters without spending a fortune.

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

I bought this filter tempted by the derisory price and the fact that it is small lightweight and that it allows me in one accessory to have several plates together, as I already have plate filters and moreover 150x150, which are really inconvenient to carry around .

The product looks really good packed inside its case, it is very light and of metallic material with the various notches indicating the ND set. Maybe on this one I would have liked it to actually say how many stops you were at without me having to do the math every time to figure out how many NDs I was at.(but maybe space doesn't allow for that).

I am happy with the purchase I made because it exceeded my expectations,
I tried it with focal length above 50mm where I didn't find any problem and the image turns out with uniform exposure without any halo, it must be said though that I also tried it only at ND400 without trying the intermediate stops so I couldn't tell what happens at lower stops; Unfortunately being in full daylight didn't allow me to do other tests..(of course if I can I will update by adding some more details).

Finally I think that in terms of quality/price there is no better around, at least for the ones I have tried and K&F improves year after year...

I enclose a few photos to make yourself aware of the product and the result....