Originally posted by baro-nite 
A fully manual lens (no "A" setting on the aperture ring) only works in M or Av mode on a Pentax DSLR. And Av mode will always leave the lens wide open when shooting, no matter what you set the aperture ring to.
Not quite true, though the gist is there. For a K-mount lens, the camera will only stop the lens down at exposure time if camera is in M mode or a lens with "A" contacts is detected**. For lenses with fully manual aperture* (say a M42 lens with the A/M switch in the M position), the lens is always stopped down so Av mode is also a possibility. In fact, the camera will default to Av mode when "A" contacts are not detected and any mode other than M is set.
Unfortunately, none of the current model Pentax cameras is capable of reliable stop-down metering in Av mode regardless of the lens mounted. As a result, M mode with green button metering is recommended for all non-A contact lenses.
Steve
* Automatic vs. manual aperture is not the same as presence of the "A" position on the aperture ring. The "A" indicates support for automated aperture control which is what Pentax calls body control of the aperture setting. Automatic aperture, on the other hand, means the aperture remains open until it closes "automatically" at exposure time.
** The "A" contacts must be present and the aperture ring in the "A" position in order for the lens to be detected as an "A" contact lens.
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