Canon EOS 20D - Manual (Page 78)

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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. The smaller the aperture (larger f/number), the
wider the depth of field (range of acceptable focus). The larger the
aperture (smaller f/number), the narrower 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 With a small aperture