The Leica R lenses can be designed using M lens designs, but only for long focal lengths because the last glass element of rangefinder telephoto lenses is far away from the film plane and 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.
Structurally, the quality of the Summicron R50mm lens should be comparable to that of the M50mm lens. However, due to the reflex mirror, I think the last glass element of the R50mm lens is still further away from the film plane than that of the M lens, so minor adjustments need to be made structurally. Wide-angle lenses use symmetric designs, with the rear group very close to the film plane, which greatly improves the imaging quality. Symmetric design is like eating an apple and having a core left over, with both ends roughly symmetrical but slightly different in size. However, in SLR cameras, the other half of the apple core will hit the reflex mirror. Therefore, Leica’s 35mm wide-angle lenses no longer use symmetric design to achieve Leica standards. The R lenses use materials that are harsher than those used for M lenses.
The Summicron-R 35mm f/2, introduced by Leica in 1972, uses nine glass elements, while the Summilux-R 35 1.4, introduced in 1984, uses ten glass elements and a floating lens group. Such heavy use of glass elements is to compensate for the disadvantage of the distance taken up by the reflex mirror.
The R lenses are generally more bulky in appearance because the SLR structure is very different from the rangefinder structure. When the lens is attached to the camera body, as long as the shutter is not released, the aperture is fully open. Only when the shutter is released, the camera body triggers the aperture to contract. The change in aperture value for metering is also transmitted to the camera body through a lever, but the lens aperture does not contract. To achieve these linkage mechanisms, R lenses are made relatively bulky. In fact, the craftsmanship of R lenses is more complex than that of M lenses.
The Summicron R50 E43 lens is currently available at a super cheap price and has exceptional quality, making it, in my opinion, the most worthwhile 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.
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