We all knew that the 300mm f/2.8 L IS was one of Canon's very best lenses. So what could they possibly do to make it even better?
Well, one immediately noticeable thing they could do was make it a bit lighter. Whilst it hasn't gone on quite the same dramatic weight-reduction program as the Mk II versions of the 400mm f/2.8 and 500mm f/4, it's noticeably lighter than its predecessor. The Image Stabilization system is improved too, from 3 to 4 stops of anti-shake compensation. The result is that it's very easily hand holdable.
They've also given a substantial boost to its optical performance with Extenders - the new 1.4x III and 2x III versions. This means you can have a superb 420mm f/4 or 600mm f/5.6 lens which is still hand-holdable. Both these combinations will autofocus on all EOS cameras.
And of course, as you'd expect the optics, build quality, focus speed and focus accuracy are all outstanding. This is not a cheap lens, but you won't be disappointed.