AF Area Selection Modes
19-point Automatic Selection AF
All the AF points are used to focus. This mode is set automatically in Basic Zone modes.
With One-Shot AF, pressing the shutter button halfway will display the AF point(s) <S> that
achieved focus. If multiple AF points are displayed, it means they all have achieved focus. This mode
tends to focus the nearest subject.
With AI Servo AF, the manually-selected (p.105) AF point <S> is used first to achieve focus. The AF point(s) achieving focus is displayed as <S>.
When AI Servo AF mode is set with 19-point automatic selection AF or Zone AF, the active AF point <S> will keep switching to track the subject. However, under certain shooting conditions (such as when the subject is small), it may not be able to track the subject. Also, in low temperatures, the tracking response is slower.
If the camera cannot focus with the EOS-dedicated external Speedlite’s AF-assist beam, set the AF area selection mode to Single-point AF (manual selection) and select the center AF point to autofocus.
When the AF point(s) light up, part or all of the viewfinder may light up in red. This is a characteristic of AF point display (using liquid crystal).
If you set [8C.Fn II-9 : Orientation linked AF point] to [1: Select separate AF points], you can set the AF area selection mode and manually-selected AF point (or zone) separately for vertical and horizontal shooting (p.373).