Spurred by an upcoming trip to Alaska I have notionally made the jump from entry level to enthusiast by going from a faithful EOS 350 lens kit to ordering a Canon 7D kit (18-85mm lens) and a Sigma 50-500mm OS lens together with 95mm UV filter and polariser. However my lack of knowledge is already showing - why does the Sigma box contain a step-down ring ("for APS-C cameras") and is there a problem with just using the lens on an APS-C camera with a 95mm filter?