Canon EOS Rebel T1i Instruction Manual page 117

You cannot take a picture during autofocusing. Take the picture only
while the Live View image is displayed.
AF is not possible with Remote Switch RS-60E3 (sold separately).
Focus the subject.
3
Aim the AF point over the subject,
and hold down the <A> button.
The Live View image will turn off, the
reflex mirror will go back down, and
AF will be executed.
When focus is achieved, the beeper
will sound and the Live View image
will reappear.
The AF point used to focus will be
displayed in red.
Take the picture.
4
Check the focus and exposure, then
press the shutter button to take the
picture (p.106).
Using AF to FocusN
117