My Df startet to show signs of hard use today. I usually use old lenses with the little wonder. The aperture readings from the mechanical coupling has worked flawless since I started using it, but today it wouldn't read apertures under f5,6. When I turned the ring from stopped down to full opening the reading gradually went down until long after I stopped turning the ring, but stopped at f8 or f5,6. Stopped down, the f22 lenses, read f45. So to get the correct exposure I had to use ISO to compensate. Some insider DIY tips or just use NPS?