Upload & Sell: On
I had and returned a Ziess 15. I have a hard time finding any UWA landscape scenes that don't require shift to keep level. Very nice lens but my 17TSE was a better choice for most scenes I encountered.
After I returned it, I got a 16-35 for the versatility of a zoom on those occasions when I could shoot level or near level and a 14 Samyang for when I really needed 14. (not often)
The 16-35 is my most used WA lens because it will AF and I never got good at MF. In fact that was also a problem with the Zeiss, and hard stop infinity did not seem to work either. Funny, the Samyang can be focused to a preset distance on the scale, the Ziess seemed to be a bit shy of infinity at hard stop. The 16-35 in AF using live view focus nails it every time.
I use the 17TSE when I need shift. Its sharper than the 16-35, but only when I nail manual focus. Not often.
The ability to use screw in filters on a 15 is not a big deal, I seldom use polarizers below 35mm because of the uneven sky problem. I do use ND grads however and they don't work as screw ins. My 17TSE takes any filter in my system with my homemade filter holder and lee system. I use the same filters on all my lenses.
The Ziess was a great starlight lens, but in my opinion, there is still too much noise for large printable images at high ISO on my 5D3. Star images look great at web size but are usually too dark to print. If it won't print well I am not very interested.