Canon EOS REBEL T8i Advanced User's Manual page 209

Shooting With FE Lock
The background or other factors may make subjects brighter or darker in flash photography
with subjects near the edge of the screen. In this case, use FE lock. After setting the flash
output for the appropriate subject brightness, you can recompose (put the subject toward the
side) and shoot. This feature can also be used with a Canon EL/EX series Speedlite.
* FE stands for Flash Exposure.
1.
Manually raise the flash.
2.
Focus on the subject.
Press the shutter button halfway and look in the viewfinder to check
that the [ ] icon is lit.
209