Canon EOS 30D Instruction Manual page 88

Av
Aperture-Priority AE
In this mode, you set the desired aperture and the camera sets the shutter speed
automatically to suit the subject brightness. This is called aperture-priority AE.
A higher f/number (smaller aperture hole) will make more of the foreground and
background fall within acceptable focus. On the other hand, a lower f/number
(larger aperture hole) will make less of the foreground and background fall within
acceptable focus.
* <
Av
> stands for Aperture.
With a large aperture opening With a small aperture opening
Set the Mode Dial to
<Av>.
Set the desired aperture.
While looking at the LCD panel, turn
'the
<B>
dial.
It can be set in 1/3-stop increments.
Focus the subject.
Press the shutter button halfway.
The shutter speed is set
automatically.
Check the viewfinder display and
shoot.
As long as the shutter speed is not
blinking, the exposure will be correct.