I have purchased a S-36 Extension tube for an SQ-A on E-Bay. I have used 2 rolls of film with little success. The first was grossly over-exposed (developed as black). The second was exposed normally, but the focus was well out. I focused as normally using the view-finder surround image (the split screen center was just black). I am hoping that there is some trick to focusing that I am just not yet aware of. Being an E-Bay purchase I don't have access to a manual. Is someone able to direct me to some information so that I can sort out these problems without wasting too much film? Much appreciated.