Canon Rebel T2i EOS 550D Instruction Manual page 116

Using AF to Focus
Live Mode and u (Face Detection) Live Mode Notes
AF operation
Focusing will take slightly longer.
Even when focus has been achieved, pressing the shutter button
halfway will focus again.
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 might flicker and focusing can be difficult. If this happens,
stop the Live View shooting and autofocus under the actual light
source first.
If you press the <u> button in the Live mode, the image will be
magnified at the AF point. If focusing is difficult in the magnified view,
return to the normal view and autofocus. Note that the AF speed
may differ between the normal and magnified views.
If you autofocus in the Live mode's normal view and then magnify
the image, the focus might be off.
In the u Live mode, pressing the <u> button will not magnify the
image.
In the Live mode or u (face detection) Live mode, if you shoot a
peripheral subject and the target subject is slightly out of focus, aim the
center AF point over the subject to focus, then take the picture.
The AF-assist beam will not be emitted.
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