Canon EOS 100D Instruction Manual page 167

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 [A: 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).
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.150).
Using AF to Focus (AF Method)
167