Canon EOS 70DN Basic Instruction Manual page 266

f: Selecting the AF OperationN
AI Focus AF for Switching the AF Operation Automatically
AI Focus AF switches the AF operation from One-Shot AF to AI
Servo AF automatically if a still subject starts moving.
After the subject is focused in One-Shot AF, if the subject starts
moving, the camera will detect the movement, change the AF
operation automatically to AI Servo AF, and keep tracking the
moving subject.
When focus is achieved with AI Focus AF with the Servo operation active,
the beeper will continue beeping softly. However, the focus confirmation
light <o> in the viewfinder will not light up. Note that focus will not be
locked in this case.
AF-Assist Beam with the Built-in Flash
Under low-light conditions, when you press the shutter button halfway,
the built-in flash may fire a brief burst of flashes. It illuminates the
subject to enable easier autofocusing.
AF-assist beam will not be emitted by the built-in flash in <7>, <3>, or
<5> mode, or when [Flash firing] is set to <b> in <A>, <C>,
<2>, <4>, or <F> mode.
The AF-assist beam cannot be emitted with AI Servo AF opera ion.
The effective range of the AF-assist beam emitted by the built-in flash is
approx. 4 meters/13.1 feet.
In Creative Zone modes, when you raise the built-in flash with the <I>
button, the AF-assist beam will fire when necessary.
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