Steve Crist , Jun 28, 2006; 08:22 a.m.
I have the Canon 100-400 and it works great - even handheld. Focus is much faster than the Tamron. I used the Tamron briefly in the store and was not impressed (when compared to the Canon). Since you have a 20D the length of the Canon is more like 160-640 which should be more than adequate for birds. I don't recall the Tamrom having IS like the Canon but I might be wrong. I use a 5D and a 20D and on the 20D it really brings images in close. I recently shot an event at a remote control airplane club, and these were tougher shots to get than birding but the 20D 100-400 combination worked fine.