I have a 1987 500 c/m, and it seems to back-focus. This is problematic because I mostly use it for portraits with a 150 f/4 wide open, so DOF is pretty shallow. I understand that focus shift can be a result of the foam pads that hold the mirror breaking down, an incorrectly adjusted focus screen, or simply incorrect body length. I've also read that newer bodies were built to tighter tolerances. Would a newer 503CW focus more accurately? Would it be worth an upgrade? Thank you.