Above, joggers on the Longfellow Bridge and men near the Kendall/MIT MBTA station.
Chris lent me a mirror lens (Phoenix 500mm), which Matthew, Taylor and I tried out around Kendall Square and the Longfellow Bridge. Above, focus is on the back mirror; below, the reflections on the front glass are visible.
As well as being lightweight (no glass!) and relatively cheap (at about $100), mirror lenses have the amusing property of producing doughnut-shaped bokeh. (The optical quality doesn't seem to be as good as refractive telephotos — the later admittedly being much more expensive than the lens we tried out. The lens is also limited to f 8.0: the sensor noise visible in the late-evening image below is a resulting trade-off.)