I tested the K&F Concept 77mm variable ND filter for a while and was particularly impressed by its color fidelity and build quality. True Color technology ensures that images appear natural and don't have the yellow tint typical of many inexpensive ND filters. This is particularly useful for outdoor landscape photography.

The adjustability from ND2 to ND32 corresponds to approximately 1 to 5 f-stops, offering enough flexibility for longer exposures during the day or for working with a large aperture in bright light. The continuously variable dimming allows for fine adjustment, and the knurled grip on the frame makes the adjustment easy to hold. The aperture markings are also practical, allowing you to quickly find the desired light reduction setting.

The optics appear high-quality. Japanese glass with a multi-layer nano coating reduces reflections and better repels dirt, water, and oil. The CNC-milled frame sits securely in the hand and rotates smoothly. In backlighting, I didn't notice any annoying reflections or ghosting, and the manufacturer's claim of ultra-low reflection is noticeable in practice.

The filter is very useful for photography and video. It's well-suited for long exposures, reduces overexposure at wide apertures, and facilitates controlled filming with low shutter speeds. As with all variable ND filters, the best results are achieved when you don't constantly adjust the setting to the limit. In my tests, the so-called X-effect wasn't noticeable, but it can theoretically occur with extreme combinations of lens and maximum darkening. Overall, the filter is a very solid choice, especially if you value natural colors and robust construction.