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 <D> 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 <S> button and <A> button.
COP
Y