Which telephoto I am currently shooting with a Minolta 100-300 4.5/5.6 zoom. I am currently shooting birds locally and will be after Bighorns in a couple of months. I hope to go to Africa in about two years. I want to add a longer lens. I am thinking of a 500 and considering the Minolta 500/8 mirror or the Sigma 500/7.2 APO. Would anyone care to offer an opinion on these two or should I go with a faster 400 and bigger enlargements to get image size?