A lot could be done to improve upon the EF 35/1.4L, as can be seen in the Sigma 35/1.4 Art.
First, reduction of coma in the image periphery. The existing design has huge coma.
Second, better correction of astigmatism and field curvature. Again, these adversely affect corner performance.
Third, reduction in chromatic aberration, especially wide open. There's not as much color fringing as, say, the 85/1.2L II, but there is still room for improvement here.
Those are optical design issues that can be addressed in an upcoming revision. Other improvements include weather sealing and less vignetting (although it's actually pretty good here already, certainly better than the 24/1.4L II). I have no issues with AF accuracy and speed--these are quite excellent. The existing design is relatively old. If Sigma can do better at half the cost, why can't Canon? The fanboys will argue that Canon needs to make back the money they spend on R&D and retooling production. Or they might say that you can only get so much optical improvement for the price point. But considering that the only reason why prospective buyers would choose Canon over Sigma in this case is that the latter might have AF issues--this is not related to optical design nor is it related to production or R&D costs. Canon has their own PCBs, ICs, and AF algorithms to work with their own lenses and bodies. That's already settled technology for them. If anything, it cost Sigma more to backwards-engineer and continually have to tweak their understanding of Canon's AF systems on various bodies. So that kind of excuse-making does not fly.
First, reduction of coma in the image periphery. The existing design has huge coma.
Second, better correction of astigmatism and field curvature. Again, these adversely affect corner performance.
Third, reduction in chromatic aberration, especially wide open. There's not as much color fringing as, say, the 85/1.2L II, but there is still room for improvement here.
Those are optical design issues that can be addressed in an upcoming revision. Other improvements include weather sealing and less vignetting (although it's actually pretty good here already, certainly better than the 24/1.4L II). I have no issues with AF accuracy and speed--these are quite excellent. The existing design is relatively old. If Sigma can do better at half the cost, why can't Canon? The fanboys will argue that Canon needs to make back the money they spend on R&D and retooling production. Or they might say that you can only get so much optical improvement for the price point. But considering that the only reason why prospective buyers would choose Canon over Sigma in this case is that the latter might have AF issues--this is not related to optical design nor is it related to production or R&D costs. Canon has their own PCBs, ICs, and AF algorithms to work with their own lenses and bodies. That's already settled technology for them. If anything, it cost Sigma more to backwards-engineer and continually have to tweak their understanding of Canon's AF systems on various bodies. So that kind of excuse-making does not fly.
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). The growing size of newer formulas is also not my taste anyway, nowadays you have to carry 2 pounds for a 35mm lense or 4.35 lb (!) for a 55mm OTUS. I'm willing to loose some sharpness for a really good but small 6/5 formula again. There are fantastic 50mm/35mm lense out there like the leica ones, they proove that you don't need 12 elements in 10 groups to get a tack sharp image.
