Canon EOS REBEL T6i Instruction Manual page 108

AF Area Selection ModesK
19-point Automatic Selection AF
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All the AF points are used to focus. This mode is set automatically in
Basic Zone modes (except <x>).
AF Using Color Tone Detection
Configuring the following makes it easier to focus on still human subject.
• Set the AF operation to One-Shot AF.
• Set the AF area selection mode to Zone AF (manual selection of zone) or
19-point automatic selection AF.
• Under [54: Custom Functions (C.Fn)], set [6: Auto AF point selection:
Color Tracking] to [0: One-Shot AF only] (if set to [1: Disable], basically
the nearest subject will be focused on) (p.341).
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.
In low temperatures, it may sometimes become difficult to see the AF

point display because of its characteristics using liquid crystal.
108
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.
With AI Servo AF, the manually-selected (p.106) AF
point <S> is used first to achieve focus. The AF
point(s) achieving focus is displayed as <S>.