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.