Join Date: May 2002
Location: Pittsburgh
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Posts: 31,678
450-600 is quite long. Getting 450mm+ and fast (f/4 or faster) gets quite pricey. 400+, fast, small, and light just isn't going to happen.
The 30D requires f/5.6 to autofocus. As a 2xTC adds two stops, you need a 2.8 lens. Unless you want to carry around a king-kong 300/2.8 or 400/2.8, there are a few options:
The 70-200/2.8 is one of Canon's best all-around lenses. I have the IS version and it's one of my favorites. With a 1.4 or 2xTC, you can get out to 280/4 or 400/5.6.
The 100-400 is almost exactly the same size. As a 2xTC impacts image quality, I use this for long reach (280+) and the 70-200 for it's native range or with a 1.4xTC. (I don't even own a 2xTC due to the impact on quality.)
If you're shooting in bright light, the 100-400IS or a 400/5.6 prime sounds like the best bet.
The other option, which I haven't used, is the Sigmonster - Sigma 50-500/6.3. Very slow at the long end, needing lots of light. But it can make the body think it's f/5.6 so AF still works