Canon EOS 750D, Basic Instruction Manual, Page: 92 / 212

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: Changing the Autofocus Operation
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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.
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.
AF Points Lighting Up in Red
AF-Assist Beam with the Built-in Flash