Canon EOS REBEL T7i Instruction Manual page 207

The compensation amount will be retained even after you set the power
switch to <2>.
A Locking the Flash Exposure (FE lock)N
If the subject is on the side of the frame and you use flash, the subject
may turn out to be too bright or dark depending on the background, etc.
Use FE lock in such a case. 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
EX-series Speedlite.
* FE stands for Flash Exposure.
Each time you press the <A> button, a preflash is fired and the

required flash output is calculated and retained in memory.
Press the <D> button.
1
 The built-in flash will be raised.
Press the shutter button halfway and

look in the viewfinder to check that
the <D> icon is lit.
Focus on the subject.
2
Press the <A> button
3
Aim the viewfinder center over the

subject where you want to lock the
flash exposure, then press the <A>
button.
 The flash will fire a preflash and the
required flash output is calculated
and retained in memory.
 In the viewfinder, "FEL" is displayed
for a moment and <d> will light up.
D Using the Built-in Flash
.
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