Canon EOS 600D - Manual (Page 134)

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Changing the Autofocus Mode
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 may 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.
Live Mode and u (Face Detection) Live Mode Notes
In the Live mode or u (face detection) Live mode, if you shoot a
peripheral subject and it 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. However, if an EX-series
Speedlite (sold separately) equipped with a LED light is used, the LED
light will turn on for AF-assist when necessary in the Live mode and u
(face detection) Live mode.