Pentax *ist IST DS - Manual (Page 122)

120
5
F
uncti
on Re
ferenc
e
You can set so that the focusing performed when the shutter release
button is pressed halfway is canceled and is performed when the 4
button is pressed.
Set [2 (Enable AF)] in [OK btn when shooting] in the [A Custom] menu.
(p.100)
Use the 4 Button to Focus on the Subject
Select [3 (Cancel AF)]. \ appears in the viewfinder while the 4 button is
pressed. Autofocus does not activate when the shutter release button is
pressed. This is useful when you wish to temporarily use manual focus while
using autofocus.
You can focus using the focus ring and release the shutter while 4 button is
pressed when using a lens compatible with Quick Shift Focus. (Let go of the
4 button to immediately return to AF mode.)
Only use this function with lenses that are compatible with Quick Shift Focus.
OK btn when shooting
Center of AF Point
Enable AF
Cancel AF
AF with shutter release
button disabled so AF
performed with OK button