Whaasup you guys . . . a friend of mine email me this craiglist auction from a guy who is selling a 70-300mm F4-5.6 DG MACRO sigma lens for my Alpha 200 I am not buying . . . I just want to know what exactly are this kind of lens used for ?? and what is the main difference between that lens and my bundled 3.5-5.6/18-70 lens ?? Please guys explain as if you were talking to your little nephew , please . . . thanks !! p.s. He wants $45.00 dlls for the lens , is that good ?? is it better to have and don't use it , than need it and not have it ??