I just had a look at the lens schematics of the Leica 50mm Summicron-M and the Zeiss Planar 50 ZM, just out of curiousity. They’re remarkably similar. Now, I’m not a lens expert, but I find it striking how close the two lenses are in terms of their optical design. But look for yourself.

Leica 50mm Summicron-M (current version) schematic. Picture © Leica Camera AG

Zeiss 50mm Planar ZM schematic. Picture © Carl Zeiss AG

Striking, innit? The main difference seems to be in the second group, where the Leica lens has two elements with plane surfaces, whereas the elements in the Zeiss lens are both curved. The rest is almost identical, as are the basic specs: 50mm focal length, f/2 aperture, six elements in four groups. Both made for M mount rangefinders, both purely spherical constructions, designed to deliver high sharpness. If we are to believe what is being said about both lenses, they’re also pretty close in overall performance.

EDIT: After reading a bit more into the matter, I realised that both lenses exhibit a so-called “double Gauss” design, that’s why they are so similar in terms of their optical construction. I also found an interesting note Erwin Puts wrote on the topic of Planar vs. Summicron:

For several generations the Planar design has tried to challenge the Summicron 50mm and never became as good. Now at last we have a lens that equals the Summicron-M 50mm and is even a trace better in the curvature of field area. The optical performance of the Planar is simply as good as that what can be expected form the Leica Summicron. The Double-Gauss design has been studied exhaustingly and it is now possible to equal but not surpass the Summicron design as long as you stay within the D-G limits. It is worth some study to note that the curved elements of the Planar bring no significant improvements in comparison to the many planar surfaces of the current Summicron.

(Source: http://www.imx.nl/photo/zeiss/zeiss/page65.html)