Canon EOS Rebel T6i Basic Instruction Manual page 92

f: Changing the Autofocus OperationK
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.
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.
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