By the way, speaking of the 100-400 and add-on items, the other day I tried a 13mm extension tube on the 100-400. I know that Canon says not to use the tubes with zoom lenses, only primes, but I was doing some macro work with a largish leaf hopper and wanted more working distance than I could get with the 60mm macro on a 20D. So, just to see what would happen, I fitted the tube to the 100-400. The result was pretty much what Canon said it would be - unusable.
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