After playing with both the Canon 100-400mm f4.5-5.6 IS and the Canon 400mm f5.6, I've decided to go with the fixed 400.
It was not an easy decision. Both lenses are very nice and in the same price range. They each have their pros and cons, which are well-discussed on internet forums. Here are some of the major points:
- versatility from 100mm to 400mm
- Image Stabilization
- close focus to 6 feet (versus 11.5 feet of the prime)
- better low-light performance thanks to the IS
- lighter and less bulky
- sharper at 400mm (though some on internet forums dispute this...my field tests showed the prime sharper, as expected)
- great image quality when shot "wide open" at f5.6
- fast autofocus (key for birds in flight)
- better built-in hood
- fewer moving parts, so fewer issues with dust, moisture, and breakdown
- better performance with a teleconverter than the zoom
Defenders of both lenses make convincing arguments:
Zoom - "The zoom is just about as sharp as the prime, so why not go with the flexibility of the zoom and the image stabilization option. It's a no-brainer."
Prime - "You're going to do 99% of your bird shooting at 400mm, and the prime is sharper at 400mm, so the prime is the way to go. It's a no-brainer."
I know I'm going to miss some shots because a bird is either too close for focus (rarely, but it will happen eventually), too close to get the whole bird in the frame, or too difficult to track in flight while at 400mm (though this will come with practice). But I went with the lens that felt great in my hands, was simply sharper, and faster-focusing! If money was not an issue, I'd probably get both!