Canon EOS Rebel T3i 18-55mm IS II Lens Kit - Manual (Page 157)

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The menu options displayed under the [n], [o], and [Z] tabs
are explained below.
[n] tab
Movie exposure
Normally, set this to [Auto].
Setting it to [Manual] enables you to manually set the ISO speed,
shutter speed, and aperture for movie shooting (p.144).
AF mode
The AF modes will be the same as described on pages 131-137.
You can select [Live mode], [u Live mode], or [Quick mode]. Note
that continuous focusing of a moving subject is not possible.
AF with shutter button during k (movie recording)
When [Enable] is set, AF is possible during movie shooting.
However, continuous autofocusing is not possible. If you autofocus
during movie shooting, you might momentarily throw the focus way
off or change the exposure. The movie will also record the lens
operation noise.
If the AF mode is set to [Quick mode], AF will be executed in [Live
mode].
3 Menu Function Settings
The settings under the [n/o/Z] menu tabs will take effect
only in the <k> mode. They will not be applied in shooting modes
other than the <k> mode.
The AF mode setting will also be applied to Live View shooting.
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