Lenses on modern digital cameras are increasingly being optimized for sharpness and imaging performance, which is of course a good thing, but on the other hand it also creates a dilemma: subjects often appear harsh and high-contrast, and images somehow lack a certain charm. Effect filters like the Nano-K Black Diffusion from K&F Concept are designed to soften this modern look, make subjects appear a little softer, and make highlights appear diffuse and somewhat softer, and they are particularly popular with cinegraphers. Photographers also benefit from such filters, as portraits, for example, take on a pleasant, somewhat dreamy look without losing too much sharpness. The diffusion effect is best shown when a light source, such as a window or lamp, is in the field of view. You will have to experiment a bit at first to find the optimal filter strength for the desired look. The K&F Nano-K Black Diffusion 1/4 that I tried out is, in my opinion, ideal for gaining initial experience, as the 1/4 filter thickness already brings out the diffusion effect very well, while still maintaining sufficient sharpness and contrast. In terms of feel and quality, there is nothing wrong with the K&F Black Diffusion. The filter is very well made and has a flat profile to avoid vignetting. The thread is cleanly machined and smooth. I did not notice any noticeable color shifts or vignetting. Tip: I always buy filters based on the largest lens diameter I have and then adjust the filter to the respective lenses on which I intend to use the filter using step-up rings. This has the advantage that you only have to buy filters once, and even though large filters are of course a bit more expensive than small ones, it's worth it in the long run because step-up rings don't cost much. It may also be worth buying a complete set of such adapter rings with different diameters.