Kristoffon,
The focused image on the screen needs to be the same distance from the lens as the distance to the sensor for manual focusing. The 0.3mm shim in my camera must provide the exact required spacing when the screen is pressed against the pentaprisom sub-frame. Very interesting.
Dave
Originally posted by kristoffon Screen thickness is definitely critical. So if some micrometrical difference in thickness between sides of my shim caused focusing problems I'll assume focusing screen thickness will be very critical.