Canon EOS 100D Instruction Manual page 169

Notes for u+Tracking / FlexiZone - Multi / FlexiZone - Single
AF Operation
Focusing will take longer than with [Quick mode].
Even when focus has been achieved, pressing the shutter button
halfway will focus again.
You cannot focus a moving subject continuously.
The image brightness may change during and after the AF
operation.
If the light source changes while the Live View image is displayed,
the screen may flicker and focusing may be difficult. If this happens,
exit Live View shooting and autofocus under the actual light source.
When [FlexiZone - Multi] is set and you press the <u> button (or
tap < > on the screen), the center of the selected zone (or image
center with automatic selection) will be magnified. If you press the
shutter button halfway, the display will return to normal and the
camera will focus.
When [FlexiZone - Single] is set and you press the <u> button (or
tap < > on the screen), the area covered by the AF point will be
magnified. Press the shutter button halfway to focus while in the
magnified view. This is effective when the camera is attached to a
tripod and you need to attain very precise focus. If focusing is
difficult in magnified view, return to the normal display and use AF.
Note that the AF speed may differ between the normal and
magnified views.
If you magnify the view after focusing with [FlexiZone - Multi] or
[FlexiZone - Single] in the normal view, precise focus may not be
achieved.
If [u+Tracking] is set, magnified view is not possible.
Using AF to Focus (AF Method)
169