Quote Originally Posted by Roger Clark View Post
Ian,
Check lensrentals.com: they have MTF charts for all (most) tele lenses. I look at the sigma zoom MTF charts from another thread here, and like typical zoom (not offending an manufacturer here), it is soft compared to the fixed focal length lenses. So if soft and slower AF, then one has to weigh those factors against continuing to save for a better lens (again independent of manufacturer).

In general, telephoto zooms have been softer and AF slower, but we can hope that will change with new designs. The Nikon 200-400 narrowed the gap between zoom and fixed FL lenses; will the new Canon? We need the gap narrowed and the price reduced.

Roger
Again we get to the "How sharp do your images need to be under a microscope and why?" question. I have seen the RAW files that Robert O'Toole creates of birds in flight and action with various Sigma zooms. They are astounding. And sharp. And the Canon 70-200 f/2.8 II L IS trumps the in general comment. At least for me. But I don't use a microscope. :) And I have never once in my life looked at an MTF chart. There are many who say that MTF charts should be viewed as advertising only as they are created by the guys who make the cameras....