40D focus oddity and 'solution'.

I was idling away a few minutes this evening sitting in the armchair taking pictures of things around the lounge and thought what the heck is wrong the AF was not working. It seemed very erratic, it would focus one moment and then not the next and it seemed to be related as what the 'last' item I chose to take the picture of i.e. the distance away from me.

Scratches head! then I recalled I had set C.FnIII - 1 Lens drive when AF impossible. Well I am trying to recall when I set it was it before or after my trip to the park to see the deer??? If before that may explain why some images were OOF/mis focuses but hang on this was not challenging the C.Fn as the change in distance was negligible!

However, back to the point the reason I set the C.Fn was because there were instances with my 70-300 IS attached to my 350D when it would hunt across the whole focus range and I thought what a good C.Fn this is on the 40D

Turning it off solved my 'problem' but just how usefull is this C.Fn and does it work on all zoom/tele lenses? But it had me worried for a while until I remembered setting that function, phew

Edit: meant to say this odd behaviour was or appeared to be the same in "one shot" or "AI servo"

Rude & against the rules ~ please do not slap me too hard

But with 175 views and no one commenting etc I was hoping for some feedback

Hey there BB

What is the purpose of the C.FnIII - 1 Lens drive when AF impossible??

Lens Drive When AF Impossible: If CFnc III is set and the 40D is unable to detect focus, the camera will force the lens to rake focus back and forth in a bid to detect subject distance. If FCnc III is set and the camera is unable to detect focus, the autofocus system will instead stop cold until the autofocus is disengaged, then reengaged (by releasing then pressing the rear AF-ON button, for example).

emtp563 wrote in post #4108914
Lens Drive When AF Impossible: If CFnc III is set and the 40D is unable to detect focus, the camera will force the lens to rake focus back and forth in a bid to detect subject distance. If FCnc III is set and the camera is unable to detect focus, the autofocus system will instead stop cold until the autofocus is disengaged, then reengaged (by releasing then pressing the rear AF-ON button, for example).

Did you mean:

Lens Drive When AF Impossible: If CFnc III is set and the 40D is unable to detect focus, the camera will force the lens to rake focus back and forth in a bid to detect subject distance. If FCnc III is NOT set and the camera is unable to detect focus, the autofocus system will instead stop cold until the autofocus is disengaged, then reengaged (by releasing then pressing the rear AF-ON button, for example).

Yes, it was my understanding that if if the point of focus was lost - especially in AI servo but not just in AI servo ? - the lens would not hunt across the whole focus range if the 'you' slipped and the focusing point was on the background.

But what I was trying to describe above & I have yet to try a truly suitable AI servo subject (moving vehicle maybe) is that the lens effectively locks and cannot be made to re acquire focus unless the subject is 'close' to the original point of focus.

So what lenses work best with this function and as such will the likes of the 70-300 IS be alright when it is used under the correct conditions.

Would "you" like to experiment with it and let me know what "you" find out about its functioning?