A couple of images in a question about how to create a 'medieval look' appear to have ring-shaped bokeh.
Catadioptric (mirror) lenses can cause similarly shaped bokeh, but these images do not look like they were taken from a tremendous distance with a 400-600mm lens (the typical focal length of mirror lenses).
Another question describes similar shapes in the viewfinder of a camera, but the recorded image was not affected.
Besides mirror lenses, what else could cause the ring-shaped bokeh in these photos?
- Regarding JindraLacko's answer, I find it unbelievable that photographer put stickers in the center of their lenses to achieve this effect. I tried briefly obstructing the center of a lens to see what it would do, and the effect is quite different from the one seen in the sample images.