Canon EOS 5D Mark II Instruction Manual page 120

Using AF to Focus
Live Mode and Liveu (Face Detection) Mode Notes
AF operation
Focusing will take slightly longer.
Even when focus has been achieved, pressing the <p> button
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.
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.
In the Liveu mode, pressing the <u> button will not magnify the
image.
The external Speedlite will not emit the AF-assist beam.
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