I'm becoming a fan of K&F Concept lens filters. This is my third filter from them and so far I've been extremely impressed with the glass.

CONSTRUCTION
The filter attaches to my lens smoothly and comes off without binding. The frame is machined very well with a semi-rough coating that makes for a secure and confident grip on the filter while adjusting it. Adjustment is very smooth with quite a bit of resistance and won't change if you happen to brush up against it. You can gauge the VND effect with markings on the edge that range from "MIN" to "MAX" with a series of square dots that gradually increase in height.

VND filters tend to be thicker and can cause vignetting with shorter focal lengths. K&F's solution to this is to make the filter slightly larger than the lens diameter. This works great to prevent vignetting but it means that you can't use your existing lens cap. Using a lens hood is theoretically possible but the larger size means it'll be very difficult to set up even if it fits.

I have another K&F filter set that came with really nice machined caps. With the larger front ring diameter that doesn't fit your lens cap that would have been a great add.

TESTING

This filter is specified to produce 1.5 to 10 stops of light reduction. I measured approximately 1 1/3 to 12 stops of light reduction with no visual artifacts such as the dreaded "X" effect you get with some VND filters. The filter imparted a slight warm cast (~300K) that remained consistent throughout the range getting slightly warmer only at maximum.

K&F state that the filter markings are not calibrated. I measured the effect under relatively controlled conditions and found that the VND effect isn't linear but instead starts off slowly but increases much faster as you approach max. Because of this adjustments at higher strengths can be quite tricky.

I would have really appreciated calibrated markings to make adjustment easier, but since it doesn't have this feature I measured the approximate strength and made a chart.

MIN
Solid 1-4 = 1 1/3, 5-6 = 1 2/3, 7-8 = 2, 9-10 = 3, 11 = 4
Hollow 1 = 5, 2 = 7, 3 = 12
MAX

You can ignore the Min and Max markings since they don't produce any meaningful changes.

SUMMARY

With this filter you're getting great neutral glass that's comparable to much higher-end products at a much lower price point. Even with minor negatives it's a fantastic all around value. Highly recommended.

PROS
Minimal color cast (+300K)
Adjustable from ND3 to ND1000 (1.5 to 10 stops)
Multilayer coatings for glare reduction, resistance to skin oils and easy cleaning
Does not vignette
Graduated markings from 'min to max'
Threaded front ring

CONS
Filter is larger than the lens diameter
You can't use your lens cap or lens hood
Markings are not calibrated
VND effect is decidedly non-linear and tricky to adjust at higher strengths

My wife and I are picked up a couple electric scooters and I thought it would be good to mount our GoPro and get some video, so filters are very nice accessory that can make a big difference. This filter is just amazing well made, such high quality and beyond. Perfect all the way around. Screws on to my camera enclosure perfectly. Filter adjusts very smoothly and it's surprising how wide a range it has. i am no photography expert, so sure it will take me some practice to get it down.

Filter comes in a nice plastic case to protect it. I only wish it came with some kind of lens cover to protect the lens when it's out of the case. I can just take it off each time, but would be nice to leave it on.

Very nice filter, price is good, gladly recommend.

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.

I bought the K&F Concept ND filter for my Sony Alpha 7 III. I use the 67mm version.
The filter arrived the third day. Everything was well packed and undamaged.

The processing is really impressive. Very clean glass and no scratches or anything like that.
The intensity of the ND filter can be continuously adjusted from ND2 to ND400.
I took a few test photos with the filter and am absolutely satisfied.
The only drawback for which there is a star deduction: At the maximum filter level you have a bluish shimmering X in the photo. This is a bit annoying, but if you turn the intensity down to just below the MAX level, it disappears again.

All in all, you can't go wrong with the ND filter from K&F Concept. At a reasonable price you get a universal ND filter. I give it a clear buy recommendation and because of one shortcoming 4 stars.
I hope I could help :-)

I ordered the ND filter from K&F Concept because I already have one in a different size and am very happy with it. With variable filters, there is a risk that these vignettes will conjure up images. That is not the case here. However, the filter could not be screwed onto the thread correctly. After a short contact with the seller's customer support and description with photos, a new product was sent to me. I can't complain about that.

I have several different sized lens and several different brands of filters. Most are in this price with a couple less expensive and a couple of high priced premium brand name filters.

The majority of my filters in this price range perform like a lower priced filter. Some don’t quite fit on the lenses the way they should, some feel tight when I turn them, and most just don’t perform as a neutral density filter should.

This particular filter surprised me by surpassing my expectations. It feels solidly built. It looks good. The glass looks fine, and you can see it darken as you turn the lends. It’s waterproof, which is a nice touch!

I usually try to give a list of pros and cons in each of my reviews, but this lens filter seriously gives me nothing at all to give as a “con”. I will be buying additional filters from K & F. I see no reason to even try any others at this point.

I hope you have found my review to be helpful!

The first thing I noticed about the K&F concept 67 millimeter variable ND filter, is that it was packaged super nice, and when I picked it up out of the box it felt like a quality product. It had a super nice metal finish and the two pieces glided across each other very smoothly. This was my first time using an adjustable ND filter on my camera, but I was always looking for the extra versatility and adjustable ND filter could offer versus a fixed ND filter stuck at one particular stop.

This variable ND filter definitely did not disappoint, I love how easy it is to slap onto my lens and it's so great being able to switch and adjust the power of the ND filter with a simple twist of the lenscap. In addition to its obvious uses an ND filter, I love the fact that I feel like my lenses are more protected when this is on them as well because the glass is not exposed to the elements. Overall, I am definitely convinced about using ND filters and adjustable ones more specifically, so I would definitely recommend this to anyone who's on the fence about getting an adjustable one. This one definitely does the job.

I've been looking for an ND filter that is cheap but also of high quality for a long time, I found it here. The K&F Concept ND filter is of high quality and comes with a case, the adjustment ring turns comfortably and does not slip immediately. I have used the filter twice so far and will continue to use it and can only recommend it.

I have come to rely on K&F Concept for a number of filter types for my many lenses. They are well made with quality glass and I have yet to find any real faults or negatives in these great value filters. You would have to pay a TON more for filters that are only slightly better than these!

This variable ND3 to ND1000 filter provides highly capable ND correction from 1.5 to 10 f/stops. It is a 67 mm filter that is widely used...especially for the majority of Tamron lenses as well as a plethora of Sigma, Sony and Canons as well, which makes this filter an even better bargain.

The variable range is done simply by rotating the front dial of the filter until just the right ND level is found in the viewfinder. The filter features 24 multi-layer coatings to provide the clearest and distortion-free optics, using Japanese glass (Excellent) and precise CNC machining.

I can honestly say that this variable ND filter is almost as good as filters that cost as much as five times more. Construction is very good and the filters mount to appropriate lenses without drag, rubbing or other issues. Simple operation and fantastic results!

All in all, and especially at the value price, this set earns an easy FIVE STARS with me, and I can honestly and heartily recommend the set for your appropriate lenses.

K&F Concept been around for some time and they're a great go-to choice for value to performance. This review is for their variable ND3-ND1000 filter (1.5-10 stops). K&F does make more premium filters than this particular one, so this filter isn't their best, but it's still good. Build quality is fantastic. The inner stationary filter has two areas of knurling while the outer rotational filter is smooth. On the edges, you'll see lasered markings indicating minimum, boxes for stops, maximum, and an arrow indicating where you're at. Though this is somewhat useful, the filter doesn't always index on my lens so that is easily shows these markings. Furthermore, there aren't hard stops for the outer filter, so you can rotate it continuously. As you rotate through the full 360 degrees, you'll go through 2 full rotations of the minimum to maximum variable effect. Optical quality is on par with most of their other filters. Since you can rotate all the way through, and because this variable goes up to 10 stops, you'll run into the X effect towards the deeper end. When I tested my 77mm filter on my 24-70, it wasn't immediately obvious on the camera screen but became more apparent once I imported the photo. The filter does live up to its density claims though as I was able to achieve up to 10 stops of change. Overall, the variable ND3-ND1000 filter (1.5-10 stops) by K&F is a fantastic buy below $60 and provides great value for its performance. I've long eyed the variables by Tiffen and B+W and PolarPro but at 2x-5x the price, I couldn't justify it without even knowing if I truly needed a variable ND. If you're just getting into using ND filters and you want to play around with an entry-level cost variable that will last you a while, I strongly recommend the K&F. It runs into the same X issue found in nearly all variables without hard stops.