I was shooting spring foliage in morning light outside of the new office, and turned up an effect I'd never witnessed before:
Shooting east-ish at 8:45am, so the sun was to the upper left, in front of the lens. Note the concentric rings in the bokeh from the specular highlights of the fountain (center) and an apparent flare artifact of the same (upper left). Here are 100% crops from those regions, showing that the rings seem to follow the axis of incidence:
I can't find much about it in general searching, except (a) some speculation that a mixture of flare artifacts and reflections within the lens are being bounced into a "hall of mirrors" effect, and (b) the Tamron 17-50mm f/2.8 is one of several lenses that tend to strongly produce this effect when the lighting is right.
Anyone have insight on what's actually happening?