Autofocus with ultrawide angles

[pmack]

Hey all

I was testing out how the auto focus behaved on my wide angle.
A 10-22mm on my 7D.

I walked out to my balcony which overlooks a valley, the closest object is a tree 10 meters away off to the side.

With just the center focus point selected, it was consistantly selecting past infinity.
With all focus point selected, it was jumping sporadically between 1meter and the infinity mark.
Even with the camera still, each time i press to focus, it gives a different result.
Occasionally it would jump to 0.5 meters.

Of course black squares were coming up saying it was in focus, even though these squares were over scenery 400 meters away.

Now of course wide angle has a large depth of field so this isn't always a major problem, but what i want to know is if this is normal behaviour, or if there is a problem with my camera/lens.

I tested it out with an old 350D and got simillar results.
Is this just how it is?

Thanks!