I would love some suggestions. At work I shoot with a Canon EOS 20D. My default lens is my Canon 28-135mm with IS, my zoom lens is a Sigma 70-300 DL Macro Super. I have been given the chance to put a new lens request into our department's budget request. I would like a big telephoto lens. I shoot a lot of nature photos for backgrounds used in mailings, and also a lot of student activities where being able to stay back from the action and still get shots of students' faces is key. The EF 400mm f/4 DO IS USM, is what I am looking at, but then I have never had a chance to use a lens like this before. Any comments or advice would be appreciated.