There are three 50 mm lenses for Leica M that many Leica aficionados are interested in: the classical Summicron lens from 1979, the Apo-Summicron-M FLE from 2012 and the Summilux-M FLE ASPH from 2004. Fifty millimeter lenses are still quite popular and exhibit excellent characteristics. I am most interested in the performance for very fine textural details. The 40 lp/mm are a good benchmark for the crisp rendering of fine detail, especially at high MTF values. The classical rule of optical designers (50% contrast at 50 lp/mm) is still a very good rule and several recently introduced lenses for the AF Leica cameras with the L-mount show this level of performance. The M lenses can be used on older cartridge loading models from M3 to M7 and modern versions (M-A = M4P; MP = M6) and there are films available that can capture extremely fine detail. So I used the limits of 80 lp/mm and 160 lp/mm on the Zeiss K8 equipment. I am less interested in the popular notion of bokeh (unsharpness before and after the plane of sharpness) because this is a very subjective impression. I prefer hard numbers to base my conclusions on. Let me be clear: 160 lp/mm would require points with a dimension of 0.003 mm or a sensor with a pixel pitch of 3 micron to record this fine detail. This is a sensor of more than 88 megapixel. None of the three lenses would be able to perform at this level. The 80 lp/mm are a different matter. Here all three lenses can cope with this very fine detail. A few figures first: 80 lp/mm require points with a dimension of 0.006 mm or a sensor with pixel pitch of 6 micron. That is equal to 24 Megapixel, the current sensor characteristics of the M. (the same pixel pitch is also available in the forgotten M8/M8.2). So a sensor with more pixels than the current 24 Megapixel makes only sense for selective enlargements. Let us look more closely at the numbers. At 40 lp/mm the SX at f/1.4 is almost as good as the Summicron-classic at f/2. This Summicron lens is however surpassed by the APO-Summicron at the same aperture. The difference is on average ten percentage points more contrast for the Apo-version. The center is for all three lenses much better than the edges, which is to be expected. Modern design rules state that a straight line from center to edge is a more desirable performance. A careful study of the numbers will also sho that there is a difference between the sagittal and the tangential planes. The more these two differ, the more blurring and color fringes you will see. At 80 lp/mm the SX at f/1.4 is still very good, but the recording of this level of fine detail depends on the illumination of the scene. The average value for contrast of 20% is the limit. This said, the performance of a high-speed lens at f/1.4 is too good to be true and sets indeed a benchmark. At f/2 the Apo-version is ahead of the classic version. The same remarks as for the SX can be made here: this level of fine detail can be handled with ease by the Apo-version and is the limiting value for the classical version. At f/4 the ranking is:all three lenses perform now at an optimum level and there is not much to choose between the three lenses reviewed here. The classic rule that at medium apertures most lenses perform in the same league is not obsolete. If a prospective buyer uses the medium apertures most of the time, the classical Summicron is a bargain! The Apo-Summicron shows the best overall performance and the SX is the most versatile of the three. Subtle differences can be found in the performance from wide open till f/4, especially around f/2.8 the critical user might find arguments for the choice to be made. It is remarkable how good the lens design was in 1980 and how much effort one needs to improve substantially on this design, either as an overall improvement (the Apo-version) or an extension to a wider aperture (the SX).
