Craig Richardson said:
Before buying this please wait for a reviewer who understands focus shift and field curvature to weigh in. Sigma has a horrible track record in regards to these areas.
For reference:
http://www.lensrentals.com/rent/canon/lenses/normal-range/sigma-50mm-f1.4-dg-hsm-for-canon
FWIW, I had one that behaved pretty much exactly how Roger wrote it up. I sent it to Sigma USA warranty a few weeks ago for "recalibration" and it was sent back with repair noted as "firmware update". Works great now at all distances after AFMA. Apparently they did a lookup table in the firmware to map focal distance and apeture and use that to fine tune focus confirmation...or maybe they were able to express it as a transfer function...who knows, but they got it right.
That said, not sure why you reference Roger's comments about the Sigma 50, they have nothing to do with field curvature or focus shift. Plus it isn't like Canon has a stellar reputation concerning those flaws. The 50L is well known for its focus shift and the 24-70L I well known for its field curvature. All lenses have some sort of flaw, even if its only size and price like the 200L f2.