[ctrout]

It looks like I'll be the proud new owner of a Super Takumar 300mm F4 manual focus M42 mount lens. I just have a quick question or two for those people familiar with it. What is the purpose of the auto/manual switch on the lens and how do I use this lens correctly on my K-mount Pentaxes? I will also be getting an original, new in box Asahi Pentax m42 to P/K adapter with this lens.

[Adam]

If you're using the lens on k mount cameras, the clutch should be set to manual. Otherwise, the aperture will never be stopped down, as this feature was only operable on open-aperture M42 bodies.

[ctrout]

How did it work on open-aperture M42 bodies? Is there some kind of mechanical interface that allows for open-aperture metering and then stops down when the shutter release like any of the more modern combos?