Leica R lenses can be designed using M lens designs, but only for long focal lengths. This is because the last glass element of rangefinder telephoto lenses is far away from the film plane, so a reflex mirror can be placed in the middle gap, for example, for 90mm and 135mm lenses. The standard 50mm lens is at the critical point.
I believe the Summicron R50mm lens is similar in construction to the Leica rangefinder and camera Summicron-M 50mm lens. However, because there’s a reflector between the DSLR lens and the film, the R50mm lens is further away from the film plane than the M lens. So, the two lenses have different imaging styles.
Because DSLR cameras are built differently from rangefinders, R lenses usually look pretty bulky. Once the lens is attached to the camera body, the aperture is fully open as long as the shutter is not released. It’s only when you press the shutter that the camera body opens the aperture. The lever transfer mechanism in R lenses makes them a bit larger than they need to be. In fact, R lenses are more complex internally than M lenses.
The Summicron R50 E43 lens is currently available at an incredible price and has amazing quality, making it, in my opinion, the best R lens to purchase. The SLR camera can focus at close range, and the closest focusing distance of the R50 is 0.5 meters, which is much closer than the 0.7 meters of the M50.
