Canon EOS 50D Instruction Manual page 122

Using AF to FocusN
You cannot take a picture during autofocusing. Take the picture only
while the Live View image is displayed.
Autofocus will not work with the release button on the Remote Switch
RS-80N3 and Timer Remote Controller TC-80N3 (both sold separately).
122
Select the AF point.
2
Press the <o > button, then
use <9> to select the AF point.
If you keep tilting <9> in the same
direction, it will toggle between
manual and automatic AF point
selection.
Focus the subject.
3
Aim the AF point over the subject,
and hold down the <p> 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.118).