Canon EOS REBEL T7i Instruction Manual page 253

Notes for AF
AF Operation
Even when focus is achieved, pressing the shutter button halfway

will focus again.
The image brightness may change during and after the AF

operation.
Depending on the subject and shooting conditions, it may take

longer to focus, or the continuous shooting speed may decrease.
If the light source changes while the Live View image is displayed,

the screen may flicker and focusing may be difficult. If this happens,
exit Live View shooting and perform AF under the actual light source
under which you are shooting.
If you cannot achieve focus with AF, set the lens's focus mode switch to

<MF> and focus manually (p.259).
If you shoot the subject at the periphery and it is slightly out of focus,

recompose to move the subject (and AF point) toward the screen center,
focus again, then take the picture.
The AF-assist beam will not be emitted. However, if an EX-series

Speedlite (sold separately) equipped with an LED light is used, the LED
light will turn on for AF-assist as necessary.
With certain lenses, it may take more time to achieve focus with

autofocus, or accurate focusing may not be achieved.
3 Focusing with AF
253