Carl Zeiss has posted a web page with a description and pictures of a one-off STL Sonnar T* 1700mm f/4 supertelephoto lens that it made-to-order for a customer to use with his Hasselblad 203 FE medium format SLR. (This would be roughly equivalent to a 920mm f/4 on 35mm format.)
Zeiss says the optic is the world's largest telephoto lens, a claim that is hard to doubt after you learn that it weighs 256 kg (564 lbs.). It uses a servo-controlled aiming and focusing system.
The lens will be on display at Photokina.