Canon 40D - Manual (Page 76)

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Select the AF mode suiting the shooting conditions or subject. In the
Basic Zone modes, the optimum AF mode is set automatically.
1
On the lens, set the focus mode
switch to <f>.
2
Set the Mode Dial to a Creative
Zone mode.
3
Press the <o> button. (9)
4
Select the AF mode.
While looking at the LCD panel, turn
the <6> dial.
X : One-Shot AF
9 : AI Focus AF
Z : AI Servo AF
Suited for still subjects. When you press the shutter button
halfway, the camera will focus only once.
When focus is achieved, the AF point which achieved focus will flash
in red, and the focus confirmation light in the viewfinder will
also light.
With evaluative metering, the exposure setting will be set at the
same time focus is achieved.
While you hold down the shutter button halfway, the focus will be
locked. You can then recompose the shot if desired.
In the Creative Zone modes, AF is also possible by pressing the
button.
f : Selecting the AF ModeN
One-Shot AF for Still Subjects