When it was introduced in 2008, the 800mm immediately became Canon's flagship (i.e. most expensive) lens - exlcuding the legendary 1200mm f/5.6 which is only made to order, if it's made at all these days. When you spend this much on a lens, you'd expect its performance to live up to its price - otherwise, what's the point? - and it certainly does. The image quality this lens delivers is simply magnificent. The build quality is exemplary, of course, and it's waterproof, of course. The Image Stabilization system gives 4 stops of anti-shake compensation, should you wish to try using this lens without a monopod or tripod. (It is possible, just about - this lens is lighter than the Canon EF 600mm f/4 L IS USM, for example - but we wouldn't recommend it.)
This lens can be used very well with a Canon Extender (teleconverter) and it will still deliver superb image quality. The 1.4x Extender makes it a 1120mm f/8 lens which will autofocus on 1D-series cameras. The 2x Extender makes it a 1600mm f/11 lens but that will be manual focus for everybody.
Our first Canon 800mm lens has a very distinguished history. We bought it from Jim Dooley, who was the fundraising director for the Bomber Command Memorial which was unveiled in London in 2012. Jim personally raised over £250,000 through the sale of aviation photos he took with this lens. We feel privileged and honoured to have this lens in our collection, and as a gesture of thanks to Jim we'll be donating £10 to the RAF Benevolent Fund every time the lens is hired.