Canon EOS Rebel XS Instruction Manual page 69

Custom Function Settings _ _s_Jz_H_
Custom Functions are organized into four groups based on the fur_ction
type: C.Fn I: Exposure, C.Fn I1: Image, C.Fn Ill: Auto focus/Drive, CFn
IV: Operation/Others
C.Fn _: Exposure
0: li3-stop
t: 1/2-stop
Sets 1/2-stop increments for the shutter speed, aperture, exposure
compensation, AEB, etc Effective when you prefer to control the
exposure in less fine increments than I/3-stop increments.
0: Auto
The flash sync speed is set aufomaflcal_y within a range of 1/200 sec to
30 sec to suit the scene's bdghtness.
t: 11200 see. (fixed}
If you use the < Av > mode (aperture-priority AE)for flash photography,
the shutter speed will be set to 1/200 sec You need not worry about
camera shake.
(However, night scene or dark backgrounds will look darker in this
mode )
t 54