My Sigma 70-300 APO DG has got quite a lot of dirt on the inside of the rear element. It probably goes some way to explain which the quality of it is pretty poor (although I know this is a budget lens anyway). If I shine a torch from the front it's very visible. Does anyone have any idea if this is fixable or is it time to replace it? Obviously I not going to try and take it apart myself and the lens is about 8 years old so out of warranty.