Last week I bought a K100D, mainly because I have some nice
Pentax lenses from my previous film cameras, both K and 42×1.
Today, while testing a non-automatic 50 mm (Pentax-M), I
noticed it is used only at full aperture.
Is there a way to use it "stop-down" or something similar? I
don't care to use it in totally manual mode or to have a
darker image in the viewfinder.
The manual, even if very detailed, is not very useful.

Thanks in advance

Cyrus (the 42×1 virus)