problem with auto-focus

[LKavaney]

My 40D seems to have developed an issue with its auto-focus. It is zooming in and out constantly and can't fix on a focus point. Any idea what is going on? thanks

 

[Crazy Dino]

Is it while focussing at everything?

Might be that what your looking at it cant find a focus point or its too dark.

 

[StewartR]

My guess is that you're trying to focus on something that doesn't have any contrasty features, so there's nothing for the AF to lock onto.

If that's wrong, would you care to give us some more details?

 

[louise-knight]

What lens are you using? Have you tried with a different lens?

Else it might be your lens that's the problem?

 

[Canon Bob]

Dust or contamination on the mirror can cause focus problems....don't attempt to clean it unless you know what you're doing though.

Bob