Canon EOS 40D - Manual (Page 88)

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In this mode, you set the desired aperture and the camera sets the
shutter speed automatically to obtain the correct exposure suiting 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.
* stands for Aperture value (aperture opening).
1
Set the Mode Dial to <f>.
2
Set the desired aperture.
While looking at the LCD panel, turn
the <6> dial.
3
Focus the subject.
Press the shutter button halfway.
X The shutter speed is set
automatically.
4
Check the viewfinder display and
shoot.
As long as the shutter speed is not
blinking, the exposure will be correct.
f
: Aperture-Priority AE
With a large aperture opening With a small aperture opening