Via 1001noisycameras Lens Reviews.

Olivier Giroux from Photoog just tested the mystical Carl Zeiss C-Sonnar T* 1,5/50 ZM on the Leica M9. The C-Sonnar is a very compact, fast normal lens that owes its size to a design similar to that of a telephoto lens — albeit with a shorter focal length in this case. It has a uniqe drawing due to its classical, spherical design, which is soft wide open and gets very sharp when stopped down. Due to this and the fact that spherical aberrations cause focus shift with this lens, it is not a lens for everyone. Olivier, though, seems to have liked it.

Read his full review here.

An alternative to the C-Sonnar is the no-longer manufactured Voigtländer Nokton 50/1.5, which is a very good lens and comes at a very moderate price on the used market. It is sharp already wide open with some slight vignetting, gets very balanced up to f/2.8 and bitingly sharp from there on. I’ve been using it on the M8 for several weeks now and have really come to like it. You can find a review of the Nokton on the M8 over at Steve Huff’s site.