Your knowledge about zeiss is a kind of "I've heard about", your statement is an insult to Zeiss QC policy.I have my own experiences on both lenses, from outside to inside, they have much higher quality than of today EX Sigma lenses.We all didn't know what is the true in blackbox of coorperation between them.As my recognition lens optics structures like vario-sonnar ( and other planar, tessar...) belongs to the Zeiss and precise mechanical base to hold optics is very high quality, made of alloy metal not like in Canon, Sigma or Tamron lenses.The part I considered look like Sigma is electro-electronic part.So I guess that in coorperation, Sigma did on electro-electronic part, and both of them made many correction to make room for that part.Believing that Sigma can make Zeiss optics is an absurd.In photography they must know who is who and when the coorperation is considered.For Sigma making the luxury lens for their SD camera is only in dream, unless the 2nd larger coorperation is considered.