Canon EOS Rebel T6i EF-S 18-55mm IS STM Lens Kit Instruction Manual - Page 342

3 Custom Function SettingsK C.Fn-8 VF display illumination You can set whether or not the AF points in the viewfinder will light up in red when focus is achieved. 0: Auto The AF points automatically light up in red under low light. 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. When AI Servo AF is set, there is no illumination in red even when focus is achieved.  When you press the or button, the AF points will be illuminated in red regardless of this setting.  The aspect ratio lines (p.120), and the grid and flicker detection set with [52: Viewfinder display] will also light up in red. C.Fn-9 Mirror lockup 0: Disable 1: Enable Prevents camera shake due to camera vibrations in the camera caused by the reflex mirror action (mirror shock), when shooting with super telephoto lenses or close-ups (macro photography). See page 163 for the mirror lockup procedure. 342