40D autofocus problem

PUREBRAD wrote in post #14163570
I posted this in the other thread already, so sorry for the dupe, but I hope this helps...
Kinda related to what Yogi Bear just said:

A few months ago, I was having a lot of issues with intermittent focus hunting on my 50D. There was no discernible pattern I could identify, and it happened with all three of my primes (28, 85, and 200). On the forum, someone suggested to look for debris on the focus screen; stating that the AF system somehow uses the focus points in the viewfinder to compute focal distance. (I still don't know if this is true or not) That said, I took a look through my viewfinder, and there was a big 'ole stinking hair cutting directly across the center focus point. Normally, I have a fair amount of debris inside the body because I change lenses quite a bit outdoors and I usually blow the body out every couple of weeks. So, having a hair in the viewfinder is something I usually disregard to the point where I don't even notice it anymore.

Anyways, I pulled the focus screen out. Blew on it, re-installed it, powered up, and camera was working properly again.

To this day, I'm not sure if the hair on the focus screen was actually the cause of the issue, or if it was just one of those "oddball" things that corrected itself or just my general ignorance.

Anyway, i hope my post helps.

It's good to know that resolves your problem. Thank you for sharing the experience. I bet there will be another person experiencing the same symptom, and looking for suggestion/solution. That's what makes this a great forum!

Thanks Rebel!

Thank you everyone for all the tips. This is a great forum for Canon users. It never fails to come with lots of useful advise. As I said earlier my problem is very similar to the one described by Citizensmith in the original post in this thread, i.e. each individual focusing point is locking OK but when all are enabled the focus starts to hunt. I have eliminated the following:
- lens problem as this is consistent with all my lenses,
- shutter button problem as it behaves the same way using AF-ON button,
- contacts have been cleaned,
- I blew everywhere I could with air blower.
I can probably live with this problem as I can get around this by selecting specific AF point but there are some occasions where I just would like to take a quick snap without loosing time on fiddling with focal points, e.g. when I run about with my energetic boys.
I don’t have another 40D to verify this but I was wondering if this is perhaps a feature of the old 9 point system (I doubt it). It can be verified in the following way: Hung a small object on a bright wall and point the camera at it in such a way that the middle focal point is not on the object but some other points are. If you enable all focal points and try to focus the lens starts to hunt. Then, if you recompose to position the object in the middle and press the shutter again it will lock. Although I cannot say this with 100% certainty but I think it was not behaving like this before. Perhaps someone with an old xxD camera could do this simple test. I would appreciate this.
I do not fancy taking the camera to the local Canon repair shop as it does not have very good reviews (overpriced and unreliable). I may have to one day though if I want to sell this one and upgrade...

Well, I suppose everyone likes a happy ending in a story, or at least some sort of ending, e.g. I smashed my camera in rage as I could not stand the focusing problem any more.
To cut the long story short my camera is back in order again. I was thinking about the symptoms and the comments posted here and finally came to the conclusion that the problem must be caused by some sort of obstruction of the central focusing point. It actually worked on its own but not as fast and reliably as the remaining points. So I started to blow more and more air onto the sensors (there is no direct access to them, only through the little windows which can be seen at the bottom while the mirror is locked up) and eventually everything clicked back to normal. "Amazing!" I said to myself in disbelief. I was just about to pack the camera and take it to the local Canon repair shop tomorrow.
Now, should I upgrade or not? This is the question. I just feel like I should keep it for a bit longer as it really has been a very reliable camera to me for almost 5 years. It sits in my hand as if it was purpose designed. I didn't get the same impression when I held 60D in the shop and I am not sure if I can stretch myself to 7D. Anyway, enough upgrade type ranting. This could go on forever...

I would give the 40D another chance, i.e. use it for another 3 months to see if the problem is gone forever. It is a very capable camera afterall.

cfcRebel wrote in post #14256783
I would give the 40D another chance, i.e. use it for another 3 months to see if the problem is gone forever. It is a very capable camera afterall.

Yes, I think I'll keep it for now. Feels like a new camera again.
I hope the problem does not come back. Probably need to be more careful when changing lenses.