The Canon FD Reflex 500mm f8 is a mirror lens, which means that it collapses what would otherwise be a very long, heavy lens into a fairly lightweight package using mirrors. Mirrors! How do they work?? There are a few downsides: there's no adjustable aperture- it's f8 or nothing- and out-of-focus highlights will end up looking a bit like donuts. But it may be worth the tradeoff to pack this much magnification into a normal-sized camera bag. In excellent condition, with only light cosmetic wear, minor internal dust.