Canon EOS Rebel XT - Manual (Page 84)

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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 larger aperture opening (lower f/number) will result in a blurred
background ideal for portraits. This occurs because a lower f/number
decreases the depth of field (range of acceptable focus). On the other
hand, a smaller aperture opening (higher f/number) will make more of
the foreground and background fall within acceptable focus. A smaller
aperture opening increases the depth of field.
* < f> stands for Aperture value.
1 Set the Mode Dial to < f>.
2 Set the desired aperture value.
¡ While looking at the LCD panel, turn
the <
6> dial.
¡ It can be set in 1/3-stop increments.
3 Focus the subject.
¡ Press the shutter button halfway.
s 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