Ken is right - the naming of Leica lenses only tells the max aperture, not the design of the lens, like it does in Zeiss, where a Planar could be f/2 or f/2.8 (for instance).
Originally Posted by gogopix
In fact, take two lenses, the 50 Summilux-M ASPH and the 75 APO-Summicron ASPH. One is a Lux and the other is a Cron, yet they are essentially the exact SAME design (the 75 has one less element). Yet, a 75 Lux and 50 Lux are vastly different. And, the current 50 Cron and the 75 Cron vastly different.
With the S lenses, the various lenses are individually unique optical designs. Here is an excerpt from my S2 review, information which was gathered in an interview with Peter Karbe, the head of optics design for Leica:
Many of the designs for the S lenses originated with R designs. The 35mm f/2.5 Summarit-S ASPH is based on the 19mm Elmarit-R (a lens that I loved on my DMR), but includes an asphere in the front optical group to reduce aberrations. The 120mm f/2.5 APO Macro-Summarit-S used the incredible 100mm APO-Macro-Elmarit-R as a foundation, but incorporated a floating element to optimize performance at all distances from close-focus to infinity (and increased maximum aperture by 1/2 a stop to boot). The 180mm f/3.5 APO-Tele-Elmar-S owes its heritage to the legendary 180mm APO-Summicron-R. Contrary to the other three lenses, the 70mm f/2.5 Summarit-S ASPH is an entirely new design, but has a similar signature Karbe modified double gauss design like the 50 Summilux-M ASPH.
I actually forgot to add at the time of writing that the 70mm Summarit-S also uses a floating element. As Jack pointed out a few posts back, lenses are optimized for a certain distance. By using a floating element, this single distance can be extended over more of the focus range, improving performance from minimum focus distance to infinity.