Canon EOS 40D - Manual (Page 86)

86
In this mode, you set the shutter speed and the camera automatically
sets the aperture to obtain the correct exposure matching the
brightness of the subject. This is called shutter-priority AE. A faster
shutter speed can freeze the action or moving subject. Or a slower
shutter speed can create a blurred effect, giving the impression of
motion.
* stands for Time value.
1
Set the Mode Dial to <s>.
2
Set the desired shutter speed.
While looking at the LCD panel, turn
the <6> dial.
3
Focus the subject.
Press the shutter button halfway.
X The aperture is set automatically.
4
Check the viewfinder display and
shoot.
As long as the aperture is not
blinking, the exposure will be correct.
s
: Shutter-Priority AE
Fast shutter speed Slow shutter speed