The two lenses are more different than one would think. The CZ is fast and as such a very useful lens for available light photography. The Sigma is wide, much wider than the CZ (there is a huge difference between 12mm and 16mm), but needs to be stopped down around f/8 to give good results.
I will just post the results at f/8 here, because they are the most interesting. Center performance first, the CZ has better contrast than the Sigma:
CZ 16-35 f/2.8 at f/8.0:
Sigma 12-24 f/4.5-5.6 at f/8.0:
And the corner performance, first the CZ 16-35 f/2.8 at f/8.0:
and the Sigma 12-24 f/4.5-5.6 at f/8.0:
The CZ is sharper, except in the extreme-extreme corner. If you look at the corner picture, you will notice that the top right is sharp and the bottom left... less so. The Sigma is noticeably sharper in the extreme corner.
However, we are looking at the pixel level here (when you click on the pictures). For prints, I would say that the two lenses are very similar. Of course, the CZ is much faster (but then the corners are even worse... which is usually not a problem for night photography anyway). The Sigma is not too shabby either!
You may also notice that the pictures open in a new window (or new tab if you use them). This make it easier to open a few pictures and flip between them for comparison. Personally, I use tabs so I just need to click on them to flip back and for between the pictures I want to compare. The name of the pictures gives lens, focal length and aperture.
