Does anyone have experience of adjusting the focussing/zoom barrel on the CZ 35-70 Vario Sonnar. Mine would focus beyond infinity and the distance readings were out all the way through. I sent it away and it has come back worse so that now the focus changes as I zoom even at infinity and will not even reach infinity focus at 70mm with the focus barrel on its stop. At 35mm it still focusses beyond infinity. I have found the adjustment screws under the rubber sleeve. Although I am good with small tools and have done many minor camera repairs in the past, in view of the high cost of this lens I would not want to start playing around unless someone can advise me what each of the three screws does.