Canon EOS Rebel T4i 18-135mm IS STM Lens Kit Instruction Manual page 156

Changing the Autofocus Method (AF Method)
When the camera does not focus the desired target subject with
automatic AF point selection, switch the AF method to zone selection or
[FlexiZone - Single], and refocus.
Depending on the aspect ratio setting, the number of AF points will differ.
At [3:2], there will be 31 AF points. At [1:1] and [4:3], there will be 25 AF
points. And at [16:9], 21 AF points. Also, at [16:9], there will be only three
zones.
For movie shooting, there will be 21 AF points (or 25 AF points if
[640x480] is set) and three zones (or 9 zones if [640x480] is set).
FlexiZone - Single: d
Since only one AF point is used to focus, you can focus the target subject.
AF point
156
Focus the subject.
3
Aim the AF point over the subject and
press the shutter button halfway.
When focus is achieved, the AF point
will turn green and the beeper will
sound.
If focus is not achieved, the area
frame will turn orange.
Take the picture.
4
Check the focus and exposure, then
press the shutter button completely to
take the picture (p.144).
Display the Live View image.
1
Press the <A> button.
The Live View image will appear on
the LCD monitor.
The AF point < > will appear. During
movie shooting, if [Movie servo AF]
is set to [Enable], the AF point will be
displayed in a larger size.