Canon EOS 5D - Manual (Page 88)

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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 (smaller 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
(larger 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.
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