Jonathan Slack:

But this is not just about Chromatic aberration, as Peter Karbe explained to me, the lack of aberrations results in much more contrast at the point of focus than in an ordinary lens, and that this contrast falls off very quickly in front and behind the point of focus. This means that an f/2 lens can appear to have the same depth of field as an f/1.4 lens (or even less in the case of the 75 f/1.4 compared to the 75 f/2 M lens).

Leica APO lenses are so good it’s ridiculous. I have two of them for the SL (35mm and 75mm). They really do look special. One day I think I’d like the APO Summicron-M 35mm but I’d have to sell my car first.

(Photo is from the original Macfilos post)