Ok, how can I know which lenses will work on the 7D? I purchased a Sigma off Ebay and the camera won't recognize it. And it isn't just not having auto focus, the camera says lens not present! I try the same lens on a Minolta film camera and it works fine.

Here is the lens:
http://cgi.ebay.com/ws/eBayISAPI.dll...tem=3864449293


And I'd like to buy one like this, but will it work?
http://cgi.ebay.com/ws/eBayISAPI.dll...868864080&rd=1

Thanks!