Quote Originally Posted by neuroanatomist
These are the reasons I absolutely prefer the 100-400mm to the 300mm f/4 - more versatility, more reach, and better IQ at 400mm (compared to the 300mm f/4 + 1.4x, not to the 400mm prime, obviously). IQ is usually worse with a TC than with a native lens, although it really seems to me that the EF 70-200mm f/2.8L IS II at 280mm f/4 rivals the 300mm f/4L!

The 100-400 isn't sharp until at least 200mm wide open. But the real thing for most of us is this:


the 300 f/4 is as sharp at f/4 as the 100-400 at 300mm f/5.6. Plus, the 300 f/4 is $500 cheaper.