An important point about autofocus (and forgive the pun).
You have a number of AF points. One in the middle, and then 2 more, or 8 more, or 40 more: whatever. Lots, on my 1Dx:
These “points” are sensors that look for focus by looking at lines and sharpening them. But did you know that some points are sensitive only to horizontal or vertical lines? That’s why, when you select one AF point, sometimes you cannot focus even though you are pointing the AF point at a nice lined surface.
The centre AF point is always sensitive to both horizontal and vertical lines. But many other AF points are sensitive to only horizontal, or only vertical lines.
What’s more, this even depends on:
- The mode you are in
- Auto or manual AF point selection
- The minimum f-number of your lens. Some points are points (sensitive to both) when used with an f/2.8 lens,. but horizontal only, or vertical only, when used with an f/5.6 lens.
So, my strong advice: Read up on how your camera does it. And if in doubt, use the centre AF point, since it is likely more sensitive and a cross-type sensor.