The Canon Mirror Lens 5200mm 1:14 is the world’s largest SLR lens. It can focus on objects between 18-32 miles away, and has has a minimum focusing distance of 120m. It weighs 100kg without its stand, so it’s not the easiest lens to carry around.
With a full-frame DSLR you get 5200mm, but using the lens on an APS-C DSLR with a would give you a focal length of 8320mm. When one of these extremely rare lenses sold on eBay, it’s said there are only three in the world, it fetched around $50,000. Here a copy of the original Canon data sheet.
Here’s a video showing this incredible lens in use.