A friend offered me a Sigma 70-300mm F4-5.6 DG MACRO, if it fit. I googled "compatible sigma lenses for Nikon D3100" and I came up with this exact lens. I now have this lens but it does not quite fit. Does Sigma make this lens to suit various cameras? or can you change the fixture on the lens to suit my camera? or do I put it on eBay? The lens is brand new and in the box, it does have FOR PENTAX AF on the box.