Canon 500 f4 auto focus problem !

[haseeb badar]

Hello ,

I have recently bought a new canon 500 f4 L IS II lens . During the field test I found out that in order to achieve auto focus, I always have to rotate the manual focus ring and thn only autofocus starts . Sometimes while auto focus is achieved in mid way it loses focus while I try to focus on different area , again I have to rotate the manual focus ring and then autofocus starts. Is there a way to reset the lens ? Or is there any other switch or button that is playing havoc ?

Regards,
Haseeb.