I finally found the time to do a comparison test at the typical group photo distances (~20m). IMAGE LINK: https://flic.kr/p/KPNwgt Canon 50mm STM vs. Sigma 17-50mm f2.8 OS
Here are the details:
Shots were made at a typical group photo distance (~20m for 50mm, ~12m for 30mm), at the sharpest overall f-number (f/8). Sigma lens was used at 50mm and 30mm focal lengths. I corrected vignetting and CA in PP, but didn't correct geometric distortions. All shots were normalized to have exactly the same exposure and colors at the center.
As one can see, the new Canon 50mm STM lens has a comparable sharpness to Sigma 17-50mm f2.8 OS at the frame's center, but is obviously sharper at the corner. (Open the link to see the image at 100%.) One can also see rather pronounced geometric distortions on the Sigma (meaning one will loose even more sharpness when correcting these distortions in PP), whereas the Canon lens is practically free of such distortions.
A side note: The Sigma lens is quite a bit warmer than the Canon lens, and a bit "pinkier" (the white balance adjustment for the Sigma in ACR is -350 K, +4).
Another side note: The Sigma's "50mm" is 8.5% wider than the Canon's 50mm - not sure who is more accurate here.Conclusion: Canon 50mm STM is a very nice group photo lens on APS-C, very sharp from the center to corners and with very little geometric distortions, substantially better than Sigma 17-50mm f2.8 OS lens.
And as my earlier infinity test demonstrated (the link
), the Canon's new lens is also better for landscape photos. And given the extremely low cost and size/weight of the new Canon's lens, it makes a perfect sense to get one even if you have a fast f2.8 standard zoom.