Canon Speedlite 320EX Instruction Manual page 26

Turning on the Light Automatically
If you use a camera that has an automatic LED light up function, the
LED light turns on automatically when taking pictures in low-light
conditions.
Set the LED light switch to <AUTO>.
The LED light turns on or off automatically according to the ambient
light when the shooting image is displayed on the LCD monitor.
The symbol < > is displayed on the LCD monitor when the LED
light is turned on automatically.
When using the LED light to shoot portraits, keep the Speedlite at least
1 m / 3.3 ft. away from the subject. Using the LED light at close range to
the person can impair the person's vision.
When the auto light setting is set, the LED light turns on during Live View
shooting or movie shooting in low-light conditions. To save battery power
of the Speedlite, set the power switch to <OFF> or set the LED light
switch to <M> when not in use.
When the batteries' remaining power is low, the LED light may not turn
on even if the flash-ready lamp is lit. If the LED light does not turn on,
replace the flash batteries.
With cameras that have an automatic LED light up function, if the flash
batteries' remaining power is low, the <
camera's LCD monitor during movie shooting may blink. If the LED light
does not turn on, replace the flash batteries.
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