Quote - Would our eagle-eyed render gods give me a reading on this render, please? It looks pretty good to me, but I have too little experience to know what to look for. Thanks.

Sample radius of 5 is saving you a lot of computation time, which is good, but imo the large radius is showing.  I think you should have more sharpness in the occlusion shadow where the corners of the small hovering square are nearest the larger square overhead, and a very large sample radius won't give you that.  It's smoother though, that's a good result otherwise.

edit: you're also not getting any obvious brightness reflected from the sphere onto the large square over it.

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