The original (old) Sonnar lenses will have medium to low levels of contrast, along with medium to high resolution, whereas the ZM Sonnar will display high contrast and high resolution.
The other major difference is the cost.
In fact, the original sonnar f/1.5 was famed for phenomenal contrast, unlike the summitar.
L1022971 by
unoh7, 1937 uncoated 5cm f/1.5 on M9
Though I would not doubt the zm50/1.5 has yet more contrast, a clean original sonnar will give high contrast: more than any 135mm lens of it's age, possibly.
The biggest "tell" of it's age is how the very sharp centers tend to degrade on the edges, even at f/8 or f/11 compared to a 50 cron etc.
These days I think sonnar, and I think unique bokeh, but back in the day a WO shot would have been the exception. Contrast at f/1.5 is much less than at f/4 (where it is basically "modern")
L1022938 by
unoh7,1937 sonnar on M9 WO
and this is where you really get some glow in some shots. Shot above is not PPed from M9 DNG.
here is same shot with nikkor 5cm f/1.4, which was a "copy" (this time it may some pp)
L1022622 by
unoh7, on Flickr
The Nikkor has it's charm, but the zeiss is a lot better for general use IMHO, and is much sharper at infinity.
Not until 1961 and the v2 50 lux was there a better all-round high speed 50.
The Canon 50/1.4 LTM is quite nice, but not sure it can match the zeiss. I have one and should test
😉
In theory the jupiter 3 is identical, but as we know, Russian builds are highly varied to say the least. Really clean German LTM copies are almost as rare as a clean, well built, Russian lens. But perfect Contax mount sonnars can be found easily for 200US, often with camera attached lol. Then you grab an Amedeo adapter and make images with your digital M or whatever
🙂