Explained in numerous e-mails to Kenko repair facility in Huntington Beach Ca. it caused my Canon 5d Mkiii to lock, using Canon 300 F4 L while micro adjust is on. Canon 70-200 F2.8 works fine.
Works with Canon 70-200 F2.8L
Locks Canon 5d Mkiii if trying to use micro focus on 300 F4L