Friend of the site Bo Zhang recently sent us his findings regarding autofocus problems he was experiencing with his Sigma 50mm f/1.4 EX DG HSM lens
. After his teardown and testing process, Zhang surmised that paint loss on a specific rotor resulted in metal shavings that jammed the autofocus motor.
From Bo Zhang:
"Now the result is clear: The HSM needed this layer of spray paint to work properly. The paint provided scortch free friction interface. I now have to order new HSM parts to get my lens fixed. But how long can a new layer of paint last?"
Download Bo Zhang's complete teardown findings here