Canon 50D/500f4 - autofocus custom function settings (1 Viewer)

Custom Function III - autofocus/drive. I usually have this set to 0 (focus search on). When I tried it on 1 (focus search off), I couldn't get it to focus at all; indeed it didn't even try - like the camera and lens didn't appear to talk to each other. I put the 17-85 lens on the 50D and it focussed fine but not with 500 f4. So I changed it back to 0. Wondered if anyone else had the same experience and could explain (very simply please!) why.

Because focus search is off, trying to focus on something that is at the other end of the scale usually fails as you have found out. I simply turn the focusing ring to achieve near focus and then there is no problem with the lens locking on. The advantage of this setting is that the lens doesnt wander when you are focusing on a BIF for example.
Adrian

Thanks Adrian. Maybe I will try again, although at the time I couldn't get it to focus on near or far subjects, but maybe it was just stuck in the middle somewhere!