Canon EOS 600D - Manual (Page 159)

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3 Menu Function Settings
Remote control
You can use the Remote Controller RC-6 (sold separately, p.261) to
start and stop the movie shooting. Set the release mode switch to
<2>, then press the transmit button. If the switch is set to
(immediate shooting), still photo shooting will take effect.
kHighlight tone priority
If [Enable] is set, highlight detail will be improved. The dynamic
range is expanded from the standard 18% gray to bright highlights.
The gradation between the grays and highlights becomes smoother.
The settable ISO speed range will be ISO 200-6400. The Auto
Lighting Optimizer will also be set automatically to [Disable] and
cannot be changed.