I am planning on buying the 400D relatively soon and am debating whether or not i should buy a zoom lens with it. My first choice would be the Tokina 80-400. My real question is... I have a Sigma 70-300 f4-5.6 APO Macro for Canon EOS SLR's that I use on my film SLR. Can I use this on the new 400D and get moderately decent results.