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In this 35mm topic I'm interested for years. I would like to have only one very good 35mm lens that it all, but struggling in a decision for one. Since years I use the Canon 35L in combination with ZE 35 2.0. The Canon gives me extremely reliable and fast AF wide open with very good sharpness and the Zeiss has more 3D, better contrast, more flare resistance (in comparison the Canon loses also a lot more contrast shooting against the sun) and excels wide open at close focus distances.
I compared both lenses with the ZE 35 1.4, and while I liked especially its bokeh and the more subtle rendering in some situations, I disliked its CA behavior, the size of the lens and that it was much more difficult to focus than my ZE 35 2.0, which is very easy to focus.
The Sigma got my attention as well and l watched hundreds of online samples with the intention to replace my two 35s with the Sigma. As good as the lens obviously is in most areas, I have a problem with liking its bokeh. And that's one of the most important points for me in a fast 35mm lens. I would like to see a direct bokeh comparison with the ZE in many different situations. Should be revealing for me.
Regarding 3D: the Zeiss certainly has a lot, even the most obvious from all lenses I ever own. In the Sigma samples I see a lot 3D, too. At least in the focus area. Regarding the overall image, I find that the in-focus-area of the Zeiss is more pronounced than in the Sigma, because the bokeh of the Sigma is very contrasty, which shifts the attention from 3D foreground to the background and therefore the foreground doesn't appear in the same way 3D as the Zeiss. That's me preliminary conclusion, but would, like you, really like to see direct comparisions of these aspects.