Catadioptric (Mirror) Lens
A long lens that folds its light path with curved mirrors as well as glass, achieving great focal length in a short, light barrel.
Measured in
mm
In depth
By bouncing light back and forth, a mirror lens fits 500 mm of focal length into perhaps 120 mm of physical length at a fraction of the weight and cost. The trade-offs are severe: a fixed aperture with no iris, lower contrast, and an obstructed central mirror that turns every out-of-focus highlight into a doughnut ring.
Examples in practice
A 500 mm f/8 mirror lens weighing 600 g renders distant fairy lights as a field of bright rings.