As this was written, both the older ZF.2 and ZE versions of the 135/2 APO-Sonnar are available at a deep discount. The optical formula is identical except for one lens element with a slightly improved coating with the Milvus version.
The Zeiss Milvus 135mm f/2* at f/2 has a lot to offer: crisp detail along with a high level of color correction can work very well to make something beautiful come together. The problem with a lens that isn’t sharp wide open is that stopping down for better results means more depth of field which can wreck the balance of sharp vs unsharp.
What I like about the Zeiss 135/2 APO-Sonnar is that it maintains its high resolving power over its entire focusing range and goes to 1:4 for near macro shooting (which is not the case with the Sigma 135mm f/1.8 DG HSM Art). This is at about 1:5.5.
Presented in Zeiss DSLR Lenses:
Includes images up to full resolution from f/2.8 through f/11.
* I am not a fan of Zeiss omitting the lens design family (Sonnar), or the APO designation from the product name. Dumping such a tradition makes no sense to me, but alas, that train has left the station.