Leica Summicron-M 1:2/50mm: 50 years at the leading edge
The Summicron lens was introduced in 1953, slightly ahead of the Leica M camera, that came in 1954. The Summicron design started in 1943 and was derived from the Summitar. In those days, the available optical glasses restricted the designer in his wishes for ever better image quality. They had to use different methods to crate improved imagery. One of the 'tricks' is to split a lens element in two separate elements. Then the incoming rays can be bend more smoothly when traveling through the glass. The Summitar design has two lenses in the front group, consisting of cemented elements. The first one was splitted into separate lenses (distance of the air lens was 0.78mm), but the glass (BK7) had a too low index of refraction. In 1947 there is a new design where the front element has a higher index. (SK2).
In 1949 the final design was derived with a smaller diatance between the front elements (0.28mm) and glass of higher index (1.69100). made by Chance Brothers in England. This glass (SBC) had a thorium oxide in its formula and was slightly radio active.
In 1954 the Glass lab of Leitz and Schott created a new glass with the same properties and without the thorium oxide: this is the wellknown LaK9.
The first generation of the Summicron had a collabsible mount and a narrow throat as it should fit the thread mount cameras.
Already in 1952 the Leitz designers were working on a second generation lens, that came available in 1956/57: the Summicron Rigid.
The Summicron Rigid
The second design had a larger distance between the front element (1,52mm), but still the narrow throat as the lens had to be mounted into the very complex dual range mount.
This lens has very good image quality, but suffers from flare (veiling glare and secondary reflections), probably caused by the thin air lens and the quality of the coating. The complex mount may be a problem too.
The Sumcron Rigid and DR are identical in their optical cells and have identical performance. At wider apertures contrast is on the low side, and resolution is reduced too. In theory one can have a very small central spot (giving high resolution) and a wide circle of halo around the spot, giving low contrast. If one could develop the film such that the circle of halo is suppressed by a very steep characteristic curve(example: document films as Agfa Copex), resolution may be increased. But for pictures with the normal tonal scale and gradation this option is not possible. in real life the Summicron generation 2 has less definition than the successors. But stopped down to 1:4 and smaller, the Summicron Rigid can still compete with current designs. Even if it cannot win the contest, is is a redoubtable contender.
The reputation of the Summicron Rigid is based in part on the excellence of the mechanics of the mount. This is indeed impressive and may be seen as a classical example of design overkill. It is very solid and gives the impression of eternal functional life. The accuracy of the mounting and and of the parts is probably not as good as the current versions. See below.
The 50 year Summicron version
Optically this version is identical to the current version, introduced in 1979. (Generation 4). I compared a Summicron #3650186 ( my own) with a Special Edition #3952197 and on the bench they performed identically. Optically the current Summicron can still claim to be the world's best 50mm lens. This is not the same as saying that the Summicron 50mm is the best lens in the world. it is a very versatile lens, that performs very well in close up and infinity settings and at intermediate distances and at every aperture. You can use it without reservation at f2 and 70cm and can expect excellent quality. In some conditions, especially when you take pictures against a large, very clear sky, that functions as a large soft box, you can see some flare (hot spots) in the middle of the image.
A comparison of the mount shows that the SE version has a different mount.The standard Summicron has an inner and an outer mount (helix) that both move relative to each other. In the new mount there are the same helix constructs, but now two pins fix and guide the movement.
The SE version has a very smooth and even movement and its mechanical engineering is of a very high level. As example: the infinity setting (collimator check) has an official tolerance of 0.02mm. This tolerance is often exceeded by Voigtlander and Konica lenses and to be honest, sometimes a Leica lens is close to the limit. The Summicron Rigids I tested had indeed tolerances that could be measured in a few hundreds of a mm. The SE version was within 0.003mm of the norm. That is 3 thousands of a mm and so a factor of 10 better than previous versions.
There will be of course a debate whether the Rigid version or the SE version is mechanically the best: for choice of material and accuracy of machining the SE wins. But the deep satin glow of the Rigid finish has its own attraction.
The current Summicron offers performance that exceeds the capabilities of most users. The SE version adds a retro-look and will fit the MP seamlessly. On my M7 the SE looks very good and has some elegance the normal version lacks.