The 400mm DO will give more reach than any other lens under 4.3 pounds and 9 inches. If money is not a problem, then go for it.





I suggest reading Bryan's review if you haven't already:


http://www.the-digital-picture.com/Reviews/Canon-EF-400mm-f-4.0-DO-IS-USM-Lens-Review.aspx


Compared to the 70-200 f/2.8 L IS, It's about an inch longer and 1.5 pounds heavier.


The primary alternative for under 4.3 pounds is the 400mm f/5.6. It lacks the f/4 maximum aperture, so it cannot take as many teleconverters (if any, depending on your body and whether you prefer to use autofocus or not). With TC, you can convert the 1-stop aperture advantage of the 400mm f/4 DO into more detail on your subject. (Not to mention the fact that a TC fits in the bag pretty easily.) More importantly, the 400mm f/5.6 lacks I.S. It's a little sharper than the DO at f/5.6, but not nearly enough to make up for a teleconverter. It's one inch longer and 1.5 pounds lighter (2.8 vs 4.3).


The 100-400 has I.S. and zoom, but isn't quite as sharp as the 400mm f/5.6, particularly with teleconverters, and the short end of the zoom would get used very little when birding.


The 300mm f/4 with a 1.4X TC is another option, but I think the 100-400 would have the same image quality and focal length without a TC.


I can't think of any other lenses that would provide similar quality in less than 4.3 pounds.