I’m considering to get a new Canon EF 70-200mm f/2.8 L IS II USM lens for my EOS 600D. OK, this lens cost 3 times as much as my camera, but I hope it will manage to capture sharp and crisp images that this lens can produce. I looked at ISO 12233 charts and noticed that 60D produces noticeably worse images that 1Ds Mark III. Why? Is my 600D going to compare to 60D? In this case the potential of this lens will never be used on my camera. Another plan I have, is to get Canon EF 2x III Extender as sometimes I would like to capture objects in a far distance and 400 mm option would be great. From review I understood that IS functionality remains, but I got confused about autofocus. Will it work or not? Or it will work at certain aperture sizes? What are your thoughts about this combination and would it be better to get just a Canon EF 100-400mm f/4.5-5.6 L IS USM Lens without extender?
Thanks in advance!