You have to get the firmware updated from a Canon repair centre. I took my 300mm f/2.8 II in and they did it on the on spot in London in about 10 minutes. It was worth the trip. The focussing was a little erratic as I found out in microadjusting the lens previously. But, now it is spot on reproducible. You need it when you are cropping to 700x700 pixels of a small bird about 30 yards away.