Pentax *istDS Operating Manual
You can focus with the following methods.
Select the area in the viewfinder to focus (focus point) from [Auto], [Select]
and [Center]. (p.123)
Set the focus mode lever to
Look through the viewfinder
and press the shutter release
The camera is focused when the shutter release
button is pressed halfway. Focus is locked on
the area shown when shutter release button is
Manually adjust the focus.
Using the Autofocus