David, If the Leica M lenses (wide ones, especially) smear then the adapter makes no difference. In case of the M Rokkor 28/2.8 (also the 40/2), the close focus adapters pose real problems. Some of the cheap Chinese adapters (they have become really excellent of late!) do wonderfully well. Maximum clearance at the end of the adapter is the one to look out for. Also, let me add here that when looked at from the rear (what the sensor sees), my adapter mounted M Rokkor shows no shiny bits. This helps with the contrast and unexpected flares.