Originally posted by reivax Since I've got your ears right now, well your eyes actually, what is stop-down metering? I've seen that term a few times, but am not sure what it is.
Stop-down metering: The lens is stopped down to the taking aperture for the meter reading. The meter "sees" exactly the same light as the film. This is the type of metering used in most M42 film cameras such as the Pentax Spotmatic. Those have a small button on the body that turns on the meter while stopping down the lens. Once stopped down, the photographer adjusts shutter speed and aperture to center a needle in the viewfinder. Once set, you turn the meter off, focus, and shoot.
Open-aperture metering: The lens is at maximum aperture for the meter reading. Supporting this feature requires either a physical coupling between the aperture ring and the camera body or control of the aperture by the body.
Your K-r supports open-aperture metering with all Pentax lenses that have electrical ("A") contacts on the mount. Since it lacks the physical coupler (due to the so-called crippled mount), you have to use stop-down metering with lenses that lack the "A" contacts. With K-mount lenses, this is done by using the green button technique. Pushing the button momentarily stops the lens down to take the meter reading. With M42 lenses, you must put the lens in manual aperture mode and manually stop the lens down to take the reading.
Steve