Canon Rebel T2i EOS 550D Instruction Manual page 250

Quick Reference Guide
Creative Zone Modes
d: Program AE
The camera automatically sets the shutter speed and aperture
in the same way as the <1> mode.
Set the Mode Dial to <d>.
s: Shutter-priority AE
f: Aperture-priority AE
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You can change the camera
settings as desired to shoot in
various ways.
Set the Mode Dial to <s>.
Turn the <6> dial to set the desired
shutter speed, then focus the subject.
The aperture will be set automatically.
If the aperture display blinks, turn the
<6> dial until it stops blinking.
Set the Mode Dial to <f>.
Turn the <6> dial to set the desired
aperture, then focus the subject.
The shutter speed will be set
automatically.
If the shutter speed display blinks,
turn the <6> dial until it stops
blinking.