40D Custom Function C.Fn III: Lens drive when AF impossible HELP NEEDED explain please

Hi there!
I am still using the old canon 40D and i can not understand 40D Custom Function C.Fn III: Lens drive when AF impossible option!
Should i have it on or off

/M

When I had my 40D I had it turned off. My understanding of this setting is that if you have it on the Lens will continue to seek to focus even though auto focus may be impossible or near to impossible due to bad light or some other condition. I found that if the 40D did not acquire focus promptly it was not going to and I saw little point in over working the USM.
I never ran into a situation where having that option turned off caused any sort of issue.

Gavin

When AF is activated in normal operation (i.e. when this CF is set to 0), the camera will attempt to find something to focus on close to the distance at which the focus ring is currently set to focus. If there is nothing to focus on at or near that distance, the lens drive will kick in and look for a subject to focus on closer than or beyond that distance. The lens will search through its entire focus range until it finds something to focus on. If nothing is found anywhere in the focus range, AF will fail.

When the CF is set to 1 and the camera cannot find anything to focus on near to the current focus distance, AF will fail immediately rather than begin searching closer to or beyond the current focus distance.

To see the difference, set the CF to 0 and position yourself at some distance from a tree. Activate AF and you will see that no matter where the focus ring is set to start with, the camera will eventually "find" the tree and lock focus. Then set the CF to 1 and set the focus ring to the minimum focus distance. When you activate AF, focus will immediately fail. Now manually focus on or near the tree. If you now activate AF, you will immediately lock onto the tree.

In general, you probably want this CF set to 0. Subjects can appear at varying distances and you want AF to "find" them regardless of the current position of the focus ring. Of course, if the desired subject is well beyond the current focus distance, AF will find it much faster if you manually prefocus closer to the subject before activating AF.

However, setting this CF to 1 can come in handy in certain situations. For example, if you are photographing birds in flight with the birds at a fairly consistent distance against a distant background of trees and you are having trouble keeping the AF point on the bird, setting this CF to 1 will stop the camera from trying to focus on the background every time you slip off of the bird. This will make it easier to regain focus on the bird. If the distance to the birds changes significantly, you will need to manually prefocus at close to the new distance before activating AF.

If you do find occasion to set this CF to 1, just be sure to set it back to 0 when you are done with the situation that called for it or you will likely miss a shot and wonder why your camera won't focus!

Thanks for the good explanation! Now i know what the setting do!

/regards Magnus