The Zeiss C Sonnar 50mm f1.5 ZM is a modern reincarnation of a classic 1930s lens. The C in its name is for classic, but it is also for compact. This is a very small lens for its speed. Being of a classic design it may not draw to everyone's tastes, however.
To quote Zeiss "The lens design and aperture geometry reflect its predecessor from the 1930s, the Sonnar 1,5/50, which was the fastest standard lens of its time. With its fast aperture, taking photographs with a beautiful ‘bokeh’ that reflects the ambiance of the golden age of rangefinder photography."
I have owned and used this lens for a long time now, but always on a Zeiss Ikon film Camera using colour negative film, so you need to bear this in mind when reading my wittering here.
This is essentially two lenses in one; from f4 and above you have a well rounded, sharp lens; from f2.8 and downwards and especially wide open at f1.5 you have the typical character of the classic 1930's Sonnar with creamy, dreamy Bokeh. That said I would not recommend this lens for high acutance look scenics and nature if you wish to work below f5.6, something like the Zeiss Planar 50mm f2 ZM or Leica equivalent would suite you better.
Then there is the much debated question of focus shift. Well Zeiss themselves say that the design itself lends it to exhibiting focus shift that will be most noticeable when the lens is used at close focus (0.9m being the closest) where the depth of focus is very shallow. Now I have never seen any evidence of focus shift myself but I use film which supposedly is much more forgiving of such things than digital and my sort of work rarely if ever calls upon me to use this lens wide open extremely close up and especially on a moving subject, plus I have the newer version which is optimised for f1.5 rather than f2.8 (purportedly) which supposedly helps.
OK that covers off the classic bit of the C in the name of the lens but there is the other side of the name i.e. compact. Considering this is a high quality, well made f1.5 lens it measures a mere 1.5" x 2" and weighs just over 8oz. Compared to the Leica Summilux equivalent which is 2.1" x 2" and weighs 16oz (silver version) the C Sonnar is a svelte featherweight. The Sonnar formulation always was a compact design which is one of its attractions.
So I come back to my opening statement. This is a compact fast lens which may not draw to everyone's tastes. At wide apertures it is a bokeh monster like the classic 1930s lens it is modelled after, providing the rounded sharpness typical of optics of that era. Stopped down, it is a well behaved lens yielding high contrast sharpness with that sought after 3D colour pop.