Aperture problem with Pentax 100mm f/4 macro

[Tony-S]

I picked up an Pentax Spotmatic F and two lenses; 50mm f/1.4 and 100mm f/4 macro. The Aperture does not stop down on the 100mm if I push the aperture pin in with either manual or auto selected on the lens, or when mounted on the camera. With the 50mm, this works just fine. The two mounts are clearly different, so I'm not sure if I'm just doing something incorrectly with the 100mm lens. Any ideas how to get the lens' aperture to work?

 

[John Koehrer]

The 100 you have is made to use wide open metering with the Spotmatic F. I don't think it matters for the operation of the aperture.
Your problem is most likely oil on the blades or housing A CLA is in order.

 

[lxdude]

Can I assume it's not set to f/4 when you push in the pin?

 

[Tony-S]

No, it's off of f/4. I've put it on a Spotmatic and a Spotmatic SPF, manual or auto aperture, trip the shutter at f/11 and it stays wide open. I'm going to have my FIL look it over later this week. He's been working on cameras (for fun) for about 45 years and has been a Pentax guy most of that time.