Canon EOS Rebel T1i Instruction Manual page 205

The maximum burst during continuous shooting is lower.
Set the [7 Custom Functions (C.Fn)] menu's [High ISO speed
noise reduction] to [Standard], [Low], or [Disable]. If it is set to
[Strong], the maximum burst during continuous shooting will decrease
(p.185).
If you shoot a subject which has fine detail (field of grass, etc.), the file
size will be larger and the actual maximum burst will be lower than the
number mentioned on page 70.
ISO 100 cannot be set.
Under the [7 Custom Functions (C.Fn)] menu, if [Highlight tone
priority] is set to [Enable], ISO 100 cannot be set. When [Disable] is
set, ISO 100 can be set (p.186).
When I use the <f> mode with flash, the shutter speed
becomes slow.
If you shoot night scenes, the shutter speed becomes slow
automatically (slow-sync shooting) so that both the subject and
background are properly exposed. If you do not want a slow shutter
speed to be set, set the [7 Custom Functions (C.Fn)] menu's [Flash
sync. speed in Av mode] to 1 or 2 (p.184).
The built-in flash does not fire.
If you shoot continuously with the built-in flash at short intervals, the
flash might stop operating to protect the flash unit.
Troubleshooting Guide
205