It has been a while since I read the manual. I should go look at the AF fail section.
Usually I have no problem with the AF.
When I do, it is usually 'pilot error' of various sorts.
Two very common ones for me are:
- Quickly changing from subject A to subject B. When I press the shutter on subject B, the lens is still changing focus from subject A, so subject B is OOF. The 2nd shot in the burst is usually in focus, as the lens has finally focused on subject by the 2nd frame. Unfortunately, it is the OOF frame 1 that I usually want.
- Similar movement as above, but in the shift, I overshoot the subject, and the AF point is on the background when I press the shutter.
My first instance of AF confusion was when I got my first dSLR, a Nikon D70.
I set it on Auto, then shot at a family party. Over half the pics were OOF.
On studying the pics, and RTFM, I figured out that in Auto mode, the camera uses "closest subject" logic to AF.
And that was the problem, on all the OOF pics, the camera focused on something that was between the subject and me.
On the dinner table group pic, it focused on the food dishes on the table, not the guests on the other side of the table.
I NEVER again used Auto mode.
My yearbook students would use the camera in zone/area focus mode, then complain that their subject was OOF.
The first time it confused me, until I figured out how to determine what AF mode was used and what the camera focused on. Then the problem was clear.
Zone AF on the Canon uses "closest subject" logic. ah ha
After that, even though it is more difficult for them, I told them to NOT use zone AF. Because MANY times their subject is farther away than other students in the frame, and the zone focus will focus on the closer student, not their subject.
But kids being kids, most of them take the easy option and use "sports scene" mode, which uses zone AF.
With both Auto mode and scene modes people have to understand HOW the AF works, or they run right into these problems, without knowing why. Then they blame the camera.
I occasionally use face recognition AF. But usually it is more problem than it is worth, for me.
In group/party environments, the face recognition will grab faces on the side or WAY behind my subjects. And I have not been able to figure out how to limit or redirect the face AF. So I stopped using it.
In a situation where there is ONLY my subjects in the frame, then it works.