You familiar with a reflector telescope? You know, mirrors and lenses instead of simply lenses? That sort of system is called catadioptric, and several manufacturers also build camera lenses in this fashion. The methodology allows for long focal lengths at significant size reduction than traditional all-lens assemblies.
A really interesting side effect, however, is the donut-shaped bokeh visible on out-of-focus portions of resulting photographs. This is because of the secondary mirror in the center of the front portion of the lens. While the Wikipedia entry claims it’s unpleasant, several photographers on Flickr seem quite adept at using this “flaw” as an advantage, as in the above picture from Julio Jiménez. Just run a search for “catadioptric bokeh” to find some other cool examples.
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