As of late 2013 this is Canon's BIGGEST lens. Not the longest focal length - that's the 800mm f/5.6. Not the most expensive - currently that's the 200-400mm f/4 with 1.4x Extender. Not even the heaviest - that's the 800mm again. (The previous 600mm was a bit of a porker, but like all of Canon's Mk II super-telephotos this one has been on a crash diet.) But no other Canon lens has a front element as big as this - a whopping 150mm - and that makes it a bit special.

Many people consider this to be the ultimate lens for photographing small birds, amongst other things. When you really need all the reach you can get, and you also want to autofocus, the 600mm lens with an extender has a slight edge over the 800mm lens. That's what the huge front element gives you.

The build quality is superb, of course. The optical performance is beyond superb, of course. You don't need us to tell you that.

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 an 840mm f/5.6 which will autofocus on all EOS cameras. The 2x Extender makes it a 1200mm f/8 which will autofocus on high-end bodies. That's the LONGEST autofocus combination Canon makes for you.