Pentax SF10 Manual page 37

If you use an F-series lens in combination with this
camera, you can employ the following exposure modes,
in addition to the Programmed AE mode, according to
the position of the aperture ring. To set the lens aperture
at [A] or any other f-stop, turn the aperture ring while
depressing the aperture-A-lock button.
[A] • Programmed AE
• Shutter-Priority AE
[Manual] •Aperture-Priority AE
• Metered Manual
• Bulb [B]