Pentax K-X Operating Manual page 4

You can set the camera so that focusing is performed when the AF/AE-L
button is pressed. Use this setting when the autofocus by pressing the
shutter release button halfway is not desired.
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1
2
Select [Enable AF1] or [Enable
AF2] in [10. AF/AE-L Button] of
the [C Custom Setting 2] menu.
10. AF/AE-L Button
2 Enable AF2
3 Cancel AF
4 AE Lock
AF is performed when the
AF/AE-L button is pressed
1 Enable AF1
2 Enable AF2
3 Cancel AF
4 AE Lock
Auto focusing is performed by using the AF/AE-L
button or the shutter release button. (default setting)
Auto focusing is performed only when the AF/AE-L
button is pressed and not when the shutter release
button is pressed halfway.
MF appears in the viewfinder while the AF/AE-L
button is pressed. Auto focusing is not performed when
the shutter release button is pressed. (Take your finger
off the AF/AE-L button to return to normal autofocus
mode.)
The exposure setting is locked when the AF/AE-L
button is pressed. (p.108)
Press the AF/AE-L button.
Auto focusing is performed.