Originally posted by dfp771 After more use with the Sigma 35mm 1.4 I decided to sell this lens and purchase the Pentax 31mm 1.8. The Sigma still occasionally mis-focuses, even under ideal conditions. I've been missing some church shots and some formal shots that I shouldn't be missing- I always take 2 or 3 shots of groups because someone is always not paying attention, blinking, or other problems. Still, I am still missing some important shots with the Sigma. I just purchased a used Pentax 31mm on eBay from Japan. This lens will be noisier but I hope it will focus correctly.
Look, you're getting an iconic lens, Dfp, with metal all over the place ... and for manual focus it's also a beauty. You would never take a shot of a group at f1.4 so I don't think you'll miss a simple half stop of aperture.
With that Sigma, they know they have AF compatibility issues so I bought the USB dock from them to do microadjustment at different distances and firmware upgrades. It works with their Art and Contemporary lines, and it can help. For some owners of the 18-35mm f1.8, it still didn't solve their problems.