Spoiler Alert: My ratings mean nothing. I haven't bought this filter yet. However, I see the question in my headline both in the Questions about the product and in several users below who ding the product because it uses two polarizers or the filter doesn't work with a CPL. The answer to the question above is misleading and those reviews below aren't fair. Above, the seller says you could add a CPL filter to this VND filter, (true) but, based on what I've read, the results of doing will be very unreliable. Here's why: all VND filters work using two polarizing elements. (Don't complain when your VND has two elements.) One element is a linear polarizer and one is circular. As you turn one, the filter darkens or lightens. Now imagine stacking a CPL filter on top of a VND filter. This really creates three polarizing filters - a linear polarizer, a circular polarizer, and another circular polarizer. That's a lot of polarization for a photographer to keep track of, especially if your VND filter is dialed down so you can't clearly see the shot, which happens with ND filters. Let's pretend you wanted to shoot waterfalls with a VND filter and a CPL filter stacked on top of it. Imagine creating this shot given what I've told you. I couldn't do it reliably. (I might get lucky.) So, if I wanted to shot an image that required both a ND filter and a CPL filter, I'd probably get much better results with both a standard ND filter, which removes the polarization, and a CPL. I hope this helps potential buyers of this item. I'm asking for this filter as a birthday gift. So I'm not a buyer yet, but I hope my wife will be.

Challenge number 1) The K&F Concept 77 mm Variable ND filters I purchased arrived later than promised and I was not able to use it for my 9 day photoshoot. Challenge number 2) It fits on my camera lens but needs time and patience to attach to the lens but it does fit. Challenge number 3) No positive stops, no detents to let you know how far you've gone (my ND filter 1.5-10 stops). In addition, if you are invested in using your lens hood while shooting, it won't fit with these ND filters.

I'll be using the ND filters on my next photoshoot and will submit an update to my review.

I'm becoming a fan of K&F Concept 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 (about 71mm on the 67mm filter I'm reviewing). 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

This is my first variable ND filter and Im very happy with it. I got the 77mm 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.

Good for home projects

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.

As others have stated, when used correctly this filter does not produce the dreaded X. Nor does it have any color cast that I can see. The quality of construction is very good, and the ring rotates smoothly without being too tight or too loose. Quality product. Glad to have it.

This one helps to keep colors true.
Have you ever tried to photograph purple sport’s uniforms? Without this filter, they often come of digitally as navy-blue uniforms. This filter is a problem solver.
Ever photograph polar bears in the snow? This filter helps keep all the shades of white clear and precise.
I label each box with what to expect from the filter.
It makes it easy to grab and go with the correct filter.
However, when I have time, I enjoy changing out the filters and shooting the same scenes with different filters. It often yields unexpected and lovely results.
Essential filter. Well worth the time and expense.

One of the better variable ND filters I've used. The build is extremely good for the price. I really like that it is a slim build and doesn't make your setup overly clunky.

This is a filter feature I have wanted for a long time but always considered a multiple filter set, not a single variable filter. To my satisfaction I found this single variable filter works very well and takes up minimum space in my camera bag. The effect given by this filter is also such that I don't have to worry about losing sharpness due to an inferior piece of glass. I use it primarily on my Canon EOS M50.