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