How's that for a title? It's almost as big as the comparison test itself. 

Here's what the test is all about: A comparison of Canon's100~400mm ISzoom with Canon new70~200mm ISzoom used with a2X extender. The 100~400mm lens is an f/5.6 when used at 400mm, and the 70~200mm lens, though an f/2.8, becomes an f/5.6 when used with a 2X extender. 

So the test seems fair. Both lens setups have a maximum focal length of 400mm when used as tested. Both lenses have a maximum aperture of f/5.6 when used at 400mm. Here then is the rational for the test, and the results.

Test RationalI have been using theCanon 100~400mm f/4.5-f/5.6L IS zoomlens since it first came out in 1999. It has been one of my most used 35mm lenses for compressed landscape and wildlife. Most of the time I use it at full extension‚ 400mm, and frequently wide-upon at f/5.6. The results have been very good. No, it's not as sharp a lens as my 70~200mm, and nowhere near as sharp as theCanon 400mm f/4 DO ISle...