Images from my old Nikkor 500mm mirror lens have a central hot spot in addition to the out-of-focus donuts which I don't like. I've switched to traditional telephotos for larger apertures, variable apertures for increased depth of field, and better image quality. Anyone who insists on those donuts can attach an opaque object to the center of a clear filter on other lenses and get somewhat the same affect. That object need not be circular, for a variety of effects.