I have a mechanical problem. Somehow the telephoto part of the lense became unscrewed from the optical part, so the lense is now in 2 pieces! I have tried screwing them together again, but the aperture & focus don't function. No obvious damage. It happened just after I had used a 2x teleconverter, but I don't know if this had anything to do with the problem. Does anybody know how to re-assemble it again???