Pentax K-R Operating Manual page 57

You can focus with the following methods.
The camera automatically focuses on the subject
AF Autofocus
when the shutter release button is pressed halfway.
MF Manual focus Manually adjust the focus.
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You can also choose the autofocus mode from AF.S (Single mode) where
the shutter release button is pressed halfway to focus on the subject and
the focus is locked at that position, AF.C (Continuous mode) where the
subject is kept in focus by continuous adjustment while the shutter release
button is pressed halfway, and AF.A (Auto) which automatically switches
between AES and AEC. The default setting is AF.A
_: Setting the AF Mode (p.125)
1
Set the focus mode lever to
AF.