Pentax ZX-5 Operating Manual page 69

Select the Spot AF Mode to critically focus on a specific
spot of the subject which is in the spot AF autofocus
frame.
2. While holding down the Spot AF button, focus on the
main subject with the Spot AF frame indicationg in
red in the illustration. When the shutter release but¬
ton is depressed halfway down, the lens automati¬
cally focuses.
How to focus
1. Hold down the Spot AF button.
When the main subject is off the Spot AF frame, use
the focus-lock technique. See page 66.