40D Cfn 3.1

Lens drive when AF impossible.

How do most people have this set?

In the manual it says that if you leave it on the slower lenses will try to refocus which will result in a pic that is totally OOF (as opposed to slightly OOF I imagine).

Has anyone experienced benefits with it ON or OFF.

I have it ON and I can't see the benefit of having it OFF.

Use the AF-ON button to focus (C.FnIV to 3)
That will separate focus from the shutter button so you control when focus happens by pressing the AF-ON button or not. When you press the shutter button it will only lock exposure.

I believe this is meant for telephoto lens that can be MF.

I already have the AF-On setup to focus (on my 10D I used the * but it all does the same thing).

and I think you set that via CFn III 4
CFn IV 3 deals with what functions you can assign to the SET button.

lostmytoy - I think you are right. The manual takes about telephoto lenses and how if you have it set to ON it may throw them grossly out of focus.

I tried it set to off and it just didn't work well with my 100-400. The way I read/understood the manual, in my teeny tiny little noobie brain, I figured "who wants the lens hunting all over the place trying to find focus." Well, my 100-400 would sometimes not even try to focus when going from being focused on a close object to trying to focus on a far object, and the other way around. A call as well as a few emails to Canon and was told this setting was for the long telephoto lenses. They recommended that it be left in the "on" setting for most other lenses. My 40D focuses just fine this way. Happy me.