Canon EOS REBEL T6i Instruction Manual page 103

AF Points Lighting Up in Red
By default, the AF points light up in red when focus is achieved in low-
light conditions. In Creative Zone modes, you can set whether to have
the AF points light up in red when focus is achieved (p.342).
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. This illuminates the
subject to help autofocusing.
The AF-assist beam will not be emitted from the built-in flash in the
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following shooting modes: <7>, <3>, <5>, and <C>.
The AF-assist beam will not be emitted with AI Servo AF operation.
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The built-in flash makes a sound when firing continuously. This is normal
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and not a malfunction.
The effective range of the AF-assist beam emitted by the built-in flash is
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approx. 4 meters/13.1 feet.
In Creative Zone modes, when you raise the built-in flash with the <I>
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button (p.166), the AF-assist beam will be emitted when necessary. Note
that depending on the setting for [4: AF-assist beam firing] under [54:
Custom Functions (C.Fn)], AF-assist beam will not be emitted (p.340).
f: Changing the Autofocus OperationK
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