Canon EOS REBEL T8i Advanced User's Manual page 687

C.Fn 9: VF display illumination
0:Auto
The AF points achieving focus in low light or with a dark subject will automatically light
up in red.
1:Enable
The AF points light up in red regardless of the ambient light level.
2:Disable
The AF points do not light up in red.
Note
When you press the <
regardless of this setting.
The aspect ratio lines, electronic level, and grid in the viewfinder will also light up in
red.
C.Fn 10: Mirror lockup
You can prevent the camera vibration blur due to the mechanical vibrations (mirror shock)
inside the camera during shooting with super telephoto lenses or shooting close-ups (macro
photography). See
0:Disable
1:Enable
> or <
Mirror Lockup for the mirror lockup procedure.
> button, the AF points will light up in red
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