I have a Sigma 30mm f1.4 that I bought originally for use on the Black Magic Camera. It does have autofocus capability. I just tried it on the C100 for the first time and the auto focus button only works when the lens is wide open.

At f1.4 the autofocus hunts and find a sharp image.
At f1.6 and higher it just doesn't do anything! It's like it's not working.

This is not a deal breaker but I am curious if this type of behavior has been observed on any other lenses?

Thanks.