The C-Sonnar Is A Wonderful Lens
The C-Sonnar Is A Wonderful Lens
There is no focus drift, there is a shift.
The C-Sonnar 50/1.5 is effectively two lenses in one.
My experience was with a Zeiss Ikon ZM body. I owned an early version.
Below about f 5.6 the C-Sonnar 50/1.5's perceived sharpness is relatively low. Yet the rendering is aesthetically appealing. For some subjects a low-level of micro-contrast is desirable. It's hard to put into words, but I use the term reportage lens to describe this lens at apertures wider than f 5.6. That is, the center renders sharper which draws viewers' attention to the subject of interest. For reportage it is common to center the intended subject of interest. I personally enjoyed the out-of-focus rendering at wider apertures. However at f 1.5 there is some coma. The coma level decreases as the aperture narrows.
At f 5.6 and above the lens' rendering changes. The perceived sharpness is excellent. Now the lens rendering aesthetic can be described as clear and crisp. The contrast is excellent. My copy performed best at f 8. Very few lenses I've used rendered as well as the C-Sonnar. Interestingly, a lens I owned that performed similarly was the Nikkor 105/2.5 AI F mount lens - also a Sonnar design.
In my opinion the C-Sonnar lens coating technology is an important factor in the perceived sharpness at narrower apertures.
I never had any luck with others focusing my rangefinder cameras with any lens. So I never worried about that.
I avoided using mechanical rangefinder cameras for close up work. So I was never frustrated by the f 2.8 C-Sonnar focus shift. I think it is fair to say that at wide apertures with subjects relatively close, focus and recompose is challenging with the C-Sonnar. In my view this is it's greatest weakness.
In the unlikely event I ever acquire another M mount camera, the first lens I would buy is the C-Sonnar 50/1.5.