What makes things interesting is, at f/5.6 and down, the Sonnar gets as sharp as the Planar.
Originally Posted by brian steinberger
As I see it, when I need extreme sharpness, it's usually for landscapes and architecture (still subjects generally). In these occasions I would stop down around f/8 anyway, so there's no discernible difference between the Sonnar and Planar.
When I shoot close to wide open, it's because a) either I shoot in available light (Sonnar wins here), b) or I want subject isolation, which means bokeh character is (probably) more important than sharpness (not to mention that in most cases, it's for portraits that I need subject isolation, so a less sharp/contrasty lens is preferable anyway).
For me, the Sonnar is a very interesting lens, that behaves as 2 different lenses: razor sharp stopped down, and pleasingly smooth with nice bokeh wide open.
And for my shooting style, that's just the kind of behaviour that I want/suits my subjects, when shooting at these apertures.
So what's the point of the Planar you might ask. Well, it's smaller, cheaper and sharper wide open.
But if i had to choose only one lens, that would probably be the Sonnar.