The C Sonnar is a wonder and very compact for such a fast lens. I don't agree that it is special purpose. I have used it on the M5 for months at a time as my only lens. It has a distinctive character even at f5.6 depending on the shot. The dreamy out of focus background in Philip's shot is part of its charm, but the extreme shallowness of the depth of field close up in portraits is part of the excitement too. This often works out better at f2 or 2.8 for at f1.5 perhaps only one eye will be sharp. Focus shift varies from copy to copy and from one user to another. Some do not notice it. Some have been maddened by it and sold the lens. One expert here has two C Sonnars: one optimized for f2.8 and the other for f1.5. I studied mine and I think it is optimized for ~f2.2. This gives me the best of both worlds, very slight front focus at f1.5, then increasing back focus from f2.8 to f4, the sharpest focus point well contained by the depth of field provided by those smaller apertures.