I know a guy who fancies himself a bird photographer. He's not bad, actually.
He showed me some photos of a fairly rare bird, in these parts. (Trumpeter swan. Itinerants that don't usually nest here.)
It was a pretty good photo.
But I took one look at it and asked him if he used a catadioptric lens.
"Catadioptric... Like a miniature telescope. With a mirror inside"
"How did you know?"
I told him that those mirror lenses are known to produce ring-shaped halos around highlights.
"Oh, you mean bokeh," he said, "I thought that was cool."
The guy knows his birds, though. Better than anybody else I know.