Quote Originally Posted by DavidEccleston View Post
Sean, did you try updating the firmware on the Sigma 50? I updated my 35, and it now works fine.
Yep. It's running the most recent firmware. It's impossible to shoot it at anything but f/1.4 and locks up the camera if you try to change apertures (the issues are a little more nuanced than that, but I don't have the camera/lens readily available at work to describe all the eccentricities exhibited by the combination).

It has no problems when connected to my EOS 5D Mark III.