I just acquired a used ZM 50/1.5 C-Sonnar, optimized at f/2.8. I'm looking forward to seeing what I can do with it.
The seller was a professional photographer who takes far better photographs than I ever will who was selling off his film equipment because of the move to digital. He was kind enough to share his impressions of the lens, so I thought I would post them here for the benefit of anyone who is considering this lens:
Please note that I used the lens with a new Zeiss Ikon M body. I always used a hood (only to protect the lens) and I never used a filter.
The lens has 2 distinct personalities.
At apertures smaller than f/4 it is a very sharp lens. But it never gives photos with a clinical look. The contrast is well balanced in my opinion In fact it handles high contrast & sharp lighting extremely well. I never noticed any flare. The lens focuses reliably near the minimum focus distance. I typically used f/5.6-f/8 in bright conditions.
At apertures below f/4 the lens behaves very differently. The way it draws is unique & lovely. The bokeh is pleasant & subtle. The color rendition and black & white tonality is mellow. At f/1.5 the vignetting is not trivial, but as you stop down it's less noticeable. The vignetting is the C-Sonnar's only flaw.
At f/1.5 the lens gives clear well-defined photos, but they are not sharp. The term soft exaggerates the lack of sharpness; the softness wide open is unlike any other fast lens I've used. I really don't know how to describe this with words. I would think to myself:this isn't sharp, but it looks good.
By f/2, the lens' characteristics change. As you know there is a focus shift below f/2.8. In my experience unless you are near the minimum focus distance, the focus shift is more accurately described as a very asymmetrical depth of field. The 1/3 rule of thumb for DOF does not apply. The DOF extends in front of the focus point.
Now we all know focusing at small distances at f/1.5 using a rangifinder is problematic. The rangefinder alignment, how the focus cam mates to the lens, & just the smallest amount of body lean between the time the focus is set and the shutter is pushed can lead to inconsistent results.
When I used the lens wide open at distances shorter than 8 feet, my success rate was good . . . but certainly not perfect. I was never more frustrated focusing this lens than I was with any other fast lens I've owned.
After reading dozens of posts about this lens over the years, I have come to believe that some Leica M focus cams do not mate well with this lens & the focus error at relatively close distances can be large. I don't know how else to explain the difference between my experience with the C-Sonnar and those who ended up being frustrated with the lens.
My advice for subjects 6 feet and further, shooting wide open in low light situations is straightforward: I simply focused a bit behind the intended focus point as the subject distance approached 6 feet. The further away the subject was, the less I even thought about focusing.
For subjects closer than 6 feet, I would bracket focusing behind the intended focus point.
The bokeh at f/2-f/4 is very pleasant to my eye. Never hesitate to use the lens in bright' harsh light. I liked how the lens worked with Kodak Porta film.
I hope that any readers here find this description of the characteristics of the ZM 50/1.5 C-Sonnar lens as helpful as I did.
