Canon EOS 100D Instruction Manual page 335

ISO 100 cannot be set.
Under [54: Custom Functions (C.Fn)], if [3: Highlight tone priority]
is set to [1: Enable], ISO 100 cannot be set. If [0: Disable] is set, ISO
100 can be set (p.301). This also applies to movie shooting (p.186).
ISO speed [H] (equivalent to ISO 25600) cannot be set.
Under [54: Custom Functions (C.Fn)], if [3: Highlight tone priority]
is set to [1: Enable], the [H] ISO speed (equivalent to ISO 25600)
cannot be selected even when [2: ISO expansion] is set to [1: On]. If
[0: Disable] is set for [3: Highlight tone priority], [H] can be set
(p.301).
The Auto Lighting Optimizer cannot be set.
Under [54: Custom Functions (C.Fn)], if [3: Highlight tone priority]
is set to [1: Enable], Auto Lighting Optimizer cannot be set. If
[0: Disable] is set, the Auto Lighting Optimizer can be set (p.301).
Even though I set a decreased exposure compensation,
the image comes out bright.
Set [z2: Auto Lighting Optimizer] to [Disable]. When [Standard],
[Low] or [High] is set, even if you set a decreased exposure
compensation or flash exposure compensation, the image may come
out bright (p.130).
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
becomes slow automatically (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 [1/200-1/60 sec. auto] or [1/200 sec. (fixed)] (p.232).
Troubleshooting Guide
335