Lenses of modern digital cameras are increasingly being trimmed for sharpness and imaging performance, which is of course good on the one hand, but in turn also brings with it a dilemma, namely that subjects often appear harsh and contrasting and images, somehow lack a certain charm.
To defuse this modern “look” a bit, to make motifs a bit softer, highlights diffuse and a bit softer, that's what effect filters like the Nano-K Black Diffusion from K&F Concept are for and are particularly popular with cinegraphs.
But photographers also benefit from such filters, as portraits, for example, get a pleasant, somewhat dreamy look without losing too much sharpness.
The diffusion effect is best when a light source, such as a window or lamp, is in the field of view.

In the beginning, you will have to experiment a bit to find the optimal filter strength for the desired look.
In my opinion, the K&F Nano-K Black Diffusion ¼, which I tried out, is ideal for gaining initial experience, as the filter strength of ¼ already makes the diffusion effect very effective, but still maintains sufficient sharpness and contrast.

Both haptically and qualitatively, 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 worked and runs smoothly.
I couldn't notice any noticeable color shifts or vignetting.

Tip: I always buy filters according to the largest lens diameter in my inventory and adjust the filter, using step-up rings, to the lenses on which I want to use the filter right now.
This has the advantage that you only need to buy filters once and even though large filters are of course a bit more expensive, like small ones, it's worth the bottom line, as step-up rings don't cost much. It may also be worthwhile to buy a complete set of such adapter rings, with different diameters.