I have a Yashica Mat 124G and it focuses beyond infinity (whatever that means). In other words I have a crisp image in WLF when the focusing knob is not turned all the way out to infinity. Is it normal? I am planning to install a Rick Oleson screen with a split image microprism to get a better focusing aid. Is it typically a drop in replacement for the stock screen or do I need to do some shimming for hitting the infinity focus accurately? What else to check to make sure that focusing is well tuned? Any advice will be highly appreciated.