Originally posted by pingflood If the Sigma was soft wide open it was probably a poor sample. It may have many flaws, but being soft isn't one of them! Then again, Sigma quality control appears to leave a lot to be desires, which is unfortunate as the good Sigma samples are really, really good.
I bought the 30/1.4 well knowing the borders and extreme corners never quite catch up to the center performance (in lab tests), but i have yet to try a Sigma lens and return it because the lens dissappointed.
Those who complain about Sigma QC should all consider the 16-50 thread, as great as Pentax gear is, they have a bad apple it seems, i think Sigma got their reputation from whatever Sigma's bad apple was, sponsored by brand fan boy touting "This is why i only buy L glass, then i know what i get." And even L lenses have their occasional bad seed. Let's just be glad the internet exists and one can find about those bad apples, too bad some people have to be the first to bite said bad apple.
All that said, i don't think the 31/1.8 is worth the extra money unless you plan on shooting it on film, it does perform better tested on a APS-C cam as far as border performance, considering you test the sweet spot. Just not twice as good and never reaches the Sigma in center resolution.
I find the 30/1.4 to be a stellar performer, considering i will want the center in focus at a wide aperture with the background blurring into oblivion. And if there was a 31/1.8 show off thread, it would be full of such shots, so why not choose the faster less expensive one?
EDIT: But let me add, if Pentax were to make a DA 30/1.4 that is anything like the 70/2.4 and cost less than a 31/1.8 or better under $500 (street), i would have to give it a try.