Canon EOS Rebel SL1 18-55mm IS STM Lens Kit - Manual (Page 129)

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A Locking the Flash ExposureN
If the subject is too far away and beyond the effective range of the flash,
the icon will blink. Move closer to the subject and repeat steps 2 to
4.
FE lock is not possible during Live View shooting.
If you set [53: Switch S / A button] to [Enable], you can switch the
functions of the button and button.
COP
Y