It's a similar technology to the split prisms in an old manual camera.
I'm not sure why you are having problems but perhaps this explains it
better:
http://en.wikipedia.org/wiki/Autofoc...sive_autofocus
"Phase detection"
lid wrote:
> I'm asking a question.
>
> I used to think that my camera or lenses had a focus problem.
>
> It's a Canon 30D and various lenses. The problem si teh same with
> all lenses.
>
> It often simply guesses wrong with autofocus.
>
> So I tested it. I tested it first with flat text material slanted
> slightly away from perpendicular to a line from the camera, and
> found unreliable autofocus. Then I tried with flat subjects
> carefully placed perpendicular to the line of sight, and it works
> quite nicely using autofocus ... or manual focus.
>
> I should add that the autofocus is set to use only the center
> focus point. The problem is not dependant on whether I'm
> using a lens that is faster or slower than f/2.8, which is,
> I believe the point at which it switches to "fine focus mode".
>
> But used on real subjects, it simply is poor at autofocus.
> Sometimes it will focus in front, sometimes in back.
>
> I read here that this camera in fact lies about the size of the
> focus areas: I had assumed that they were the size of the little
> squares in the viewfinder, but somebody says that they are
> three times larger. That could be the problem, of course, with small
> subjects.
>
> How does the autofocus work? I read about "vertical and horizontal lines"
> but it seems that the center focus point uses both.
>
> It is simply fooled by things at different distances within its
> focusing area? Does it try for an average? The front? The back?
> OR is it just junk and I'm going to have to manual focus
> on ANYTHING of importance that is not flat and perpendicular
> to the line of sight?
>
> What do pros do ... other than of course take types of picture
> where the problem does not arise ... it's not a problem
> with distant landscapes or portraits (where the person more
> than fills any possible center focus area): there the autofocus
> works OK.
>
> But say I'm taking picture of geese waddling around in the park.
> The goose's head is simply not going to fill the center focus
> spot.
>
> Oh yes ... please do not say "you should have bought Nikon".
>
> I'm very happy with the camera otherwise: a lot of the stuff I take
> is perfectly obviously not possible for autofocus at all, such
> as picture of flowers or bugs, where I have to decide "artistically"
> where to focus. But the damn thing should focus on a goose!
>
> Doug McDonald
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