I once had the Idea that one could manually mark borders between objects and enforce a “no ringing” criterion there, using this to guide an algorithm to find the optimal kernel. But I guess that’s a bit off-topic for this thread and quite a project in its own. The idea was to use e.g. the boundary between an out-of-focus background and an object in focus for this because that’s a area in the image where we can make quite solid assumptions about how the transition is supposed to look like with a correct kernel.