I have two of these variable ND filters with 55mm and 67mm filter threads. The filters make an absolutely high-quality impression, which is also supported by the packaging, in particular the storage box.
The filters have a scale on the aperture ring that starts at ND2 and ends at ND400. There are 8 stops in between. Unfortunately, the respective extension factor for each stop is not documented, so that the exposure time is difficult to calculate. Here, you have to get the best result by trying things out.

Review of "K&F Concept 67mm Magnetic Variable ND2-32 Filter (1-5 Stop) Set for Phone Lens Camera Mount Comaptible with iPhone 15 14 13 12 11 Series, Magnetic Neutral Density with Filter Holder" currently at £49.99:

I used this with my Samsung S20FE phone and it fit well even with case on. The filter is 67mm and fits over all the cameras on my phone easily. It has a nice smooth action for adjusting the ND level. It has a magnetic clip for the lens which is nice, but I would have liked a standard screw mount so other filters could be used.

It is quite expensive for what it is, especially as I can't attach other standard 67mm filters to it, but it does its job well so I can't really mark it down.

I recently got the K&F Concept 67mm Black Diffusion Mist 1 4 Effect & Variable ND2-400 (1-9 Stops) ND Lens Filter and it is incredible! This filter has adjustable levels of density, allowing me to get the exact effect I'm looking for. Not only that, but it's multi-coated with 28 layers of Nano-X Series and made with high-grade materials that feel sturdy.

I had been wanting to try a magnetic filter system for a while so I'm glad I got my hands on these. All the magnets work well and it's easy to screw on the adapter onto your lens. I personally only really use the ND filter, but the other 2 filters could stack on no problem and without any degrading in image quality or color.

I was able to look closely at high resolution RAW files of photos I took using the ND filter and they looked extremely clean. The little carrying case is a nice touch as well.

One suggestion is maybe adding a 6 stop ND filter to the set. There were a couple times where the 10 stop was just a little too much light blocking for me.

I would definitely recommend this to most photographers as a great and easy all around filter system

I am very pleased with this 67mm ND filter from K & F Concept. I am using this on a Sony 24mm 1.4 G Master lens (Bonus it will also fit on my 85mm lens). This is great for time lapse photography during the bright afternoons in Florida. Examples of the scenes I've shot are, the summer rains coming through over the water, or of the city scenes at the urban farmers markets, etc., this filter has exceeded my expectations.
I purchased at medium sized backpack from K & F 2 years ago that has been a hearty addition. The backpack has protected my gear on trips to Thailand, Hong Kong, Philippines, and Colombia. So, I figured why not try their other products. I'm looking forward to using these filters for many years to come.
Great buy at a great price.

This is tough to beat. Not only do you get a 100mm GLASS (not plastic) 10-stop ND filter, but you also get the filter METAL mount and multiple METAL lens adapters. Even if you need to buy step-up/down adapters because the ones included do not fit your lenses, this is still a bargain. Even though this comes in at well below the price of other 100mm 10-stop filters (without the mounting system), it performs excellently.

My unit doesn't produce any noticeable color cast, but I also shoot solely in RAW, so a color cast wouldn't be an issue anyway. The case for the filter is firm, so I don't worry about it very much when it's in my camera back, and the soft lining keeps the filter safe from scratches (so long as nothing coarse gets inside, of course). Better still, the filter mount features a second slot should you wish to use additional 100mm filters (e.g. graduated filters). I point out the size because this is the "standard" in these filters, rather than the more common 80-something mm ones you see in comparably-priced square ND filter bundles that use smaller, cheaper plastic filters instead.

The only issue I found, and this is really just nitpicking at this point, is that the filter mount never really feels very securely attached to the lens. The filter and mount won't fall off if the camera is moved or tilted forward, but the mechanism that attaches the mount to the lens adapter could be a bit tighter fitting in my opinion. Nevertheless, great product at a great price. Anything "better" will be considerably more expensive, and even then, how much "better" will it be?

This is my first variable ND filter and Im very happy with it. I got the 67mm version because its a very common size and I already have three lenses that it could fit. Plus with adapters, I can get my smaller lenses to work as well.
The filter. Overall its great. Metal construction with a very smooth ring.
The stops are marked on the ring and the overall construction is very good.
Again this is my first variable ND and so far I'm enjoying it very much.
Price is affordable too.

The K&F Concept ND Filter does not have a stop, it will continue to spin all the way around. Though it does spin freely, taking the camer in and out of a bag/case may change the filter intensity. Otherwise it allows for much lower f stop. I use this while filming outdoor shots.