Canon EOS REBEL T7i Instruction Manual page 443

ISO 100 cannot be set.
Under [54: Custom Functions(C.Fn)], if [4: Highlight tone priority]

is set to [1:Enable], ISO 100 cannot be set. If [0:Disable] is set, ISO
100 can be set (p.391). This also applies to movie shooting (p.308).
Expanded ISO speeds cannot be selected.
Under [54: Custom Functions(C.Fn)], if [4: Highlight tone priority]

is set to [1:Enable], the settable ISO speed range will be ISO 200 -
ISO 25600 (up to ISO 12800 for movies) even when [2: ISO
expansion] is set to [1:On]. If [0:Disable] is set for [4: Highlight tone
priority], [H] can be set (p.391). This also applies to movie shooting
(p.308).
The Auto Lighting Optimizer cannot be set.
Under [54: Custom Functions(C.Fn)], if [4: Highlight tone priority]

is set to [1:Enable], the Auto Lighting Optimizer cannot be set. If
[0:Disable] is set, the Auto Lighting Optimizer can be set (p.391).
Even if I set a decreased exposure compensation,
the image comes out bright.
Set [z2: Auto Lighting Optimizer] to [Disable]. When [Standard]

[Low] [High] is set, even if you set a decreased exposure
compensation or flash exposure compensation, the image may come
out bright (p.169).
When I use the <f> mode with flash,
the shutter speed becomes slow.
If you shoot at night when the background is dark, the shutter speed

automatically becomes slow (slow-sync shooting) so that both the
subject and background are properly exposed. To prevent a slow
shutter speed, under [z2: Flash control], set [Flash sync. speed in
Av mode] to [1/200-1/60sec. auto] or [1/200 sec. (fixed)] (p.212).
Troubleshooting Guide
443