I picked up a secondhand but very mint condition Sigma 300mm f2.8 EX HSM Lens today. Although hardly cheap, the price was extremely good compared to the new price so couldn't resist
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Initial tests show that starfields are very sharp across the whole field of my 350D at f4.0 (with pretty good results at f3.5). I am guessing the lens would be quite usable wide open for daylight photography.
Attached is a photo show the 350D connected to the 300 2.8, and for comparision the 200 f2.8 L and the standard 18-55 kit lens are shown next to it (so you can see this is not a small lens!). Anyway once I hope to do a more thorough report soon.
Terry