I don't know if the link comes up in Spanish for other viewers, but anyway, it was a bit of an eye-opener to discover that Leica M lenses were know to demonstrate focus shift which had to be compensated for by guesstimation; I had always imagined them to be better controlled, particularly since the range of different focal lengths is so much more limited than for slr systems.
Would still love an M9, though, if anybody feels stupidly generous.
Rob C
Rob,
it is in Spanish just because it's the only on-line source at this moment.
The current 35mm Summilux 35mm ASPH has strong focus shift, and you are right: I also expected those dangerous aberrations to be better controlled. It is common problem in old designs for fast lenses (the previous Noctilux also had focus shift problems), specially when used on digital sensors. The focus has to be guesstimated and it is a pain in the ass. This lens costs 3.300 euros (taxes included), this is, more than $4,000.
Digital cameras made the focus shift problem (uncorrected spherical aberration) worse because:
1) we magnify the images much more than in the film days for focus inspection.
2) filters on the sensor may introduce additional spherical aberration and other problems (chromatic aberrations, etc.)
3) film has more thickness than the photosensitive surface of sensors
The new design seems to be based on the previous one but with several improvements: the floating element and new optical formula correct the focus shift problem to tolerable (invisible) limits; improved coatings will reduce flare; and a smaller sun hood. The price will be more than $4,500.