Canon's most flexible super zoom has very good sharpness, especially stopped down one stop from wide open. Its reasonably light weight and Image Stabilization allow it to be handheld in decent lighting. The wide zoom range maximizes shooting opportunity without changing lenses. The Canon 100-400mm is a "telescoping" (a.k.a. push-pull) zoom which takes a little adjustment if you haven't used one before.
This is a compromise lens. It's not quite as sharp as the 300 and 400mm primes and has a telescoping zoom which some people don't like. Personally, I appreciate this feature since it's faster to zoom. The Canon 70-200 f/2.8L IS II is sharper than this lens from 100-200mm. Trip after trip, however, I pack up 2 or 3 "best possible" lenses only to unpack them and put a 100-400mm in my bag. This is because, although it's a compromise, it's a very good compromise. It's nearly as sharp as the primes and far sharper than a consumer lens. The huge range means getting the shot framed just right and not having to change lenses, and it's relatively small and easy to shoot hand-held.
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