The Canon 400mm f5.6 is light, sharp, and as seen below, birds love it! Seriously though, the 400mm on a camera body with a 1.6x crop factor becomes a 640mm. This type of long reach allows you to photograph birds and wildlife from a distance, and fill the frame without disturbing them. It auto focuses very fast, and comes with a removable tripod collar. Over the past several years I've used it for a wide variety of subjects including nature, sports, and air shows.
A Black Cap Chickadee checks out my gear at Morton's Wildlife Refuge in Sag Harbor, NY on December 5th, 2008. (taken with a 70-200 at f2.8)
A Female Cardinal taken with the 400mm on December 5th, 2008.
Settings f6.3, 1/250th, Flash -1, ISO 100, Tripod
A Nuthatch at Morton's Refuge taken with the 400mm lens on December 5th, 2008.
Settings: f5.6, ISO 100, 1/200, Flash -1, Tripod