I am considering purchasing a long zoom lens for football and rodeo photos and birding. Does anyone have links to reviews comparing the Canon EF 100-400mm f/4.5-5.6 L IS USM, Tamron AF 200-500mm f/5.0-6.3 Di LD SP FEC (IF) and Sigma 150-500mm f/5-6.3 AF APO DG OS HSM? Any personal opinions (like I have to ask) whether the Canon is worth the extra $500-$700? I shoot a 7D, mostly for newsprint (so everything gets halftoned down to 150 dpi).