Canon EOS 7D Instruction Manual page 145

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 AF point area will

be magnified. If it is difficult to focus 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.

Using AF to Focus
145