Originally posted by IchabodCrane Is it possible that your Sigma 18-35 started so out of focus, coupled with being an f/1.8 lens, that the PDAF sensor had nothing to pick up to drive the focus?
I do not know but the thought crossed my mind - at f2.8 (Sigma 17-50) you can sort of make out the black bars through the viewfinder) but they are hard to see if at all, at f1.8.
One the other hand, with the lens cap on the lens attempts to find something by traveling through the entire focus range - and back again.