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Either the Sigma 35/1.4 has a lot of field curvature, or Sigma haven't figured out the electronic interface with the Canon AF system. Either way, you are better off using the Canon 35mm 2.0 IS or if you really want to focus by hand and get a lot of unfocussed pictures, the Zeiss.
The Sigma electronic interface works perfect on my Canon 1DX, 1Ds3, 1D4, 1D3 and 100D bodies. And it gives me perfect in focus shots