[hatsofe]

I just received this lens which I bought on eBay and I figured that moving an aperture ring in manual mode doesn't change the aperture opening. I'm not familiar with medium format Pentax lenses but on Pentax A lenses the aperture ring actually changes the opening wether the lens attached to a camera or not. Is it different in case of 645 A lenses? I wouldn't ask such question if I could check father, but I haven't got the "645 to K mount" adapter yet. Did I buy a lemon?

[ProfessorBuzz]

This sounds normal, not a lemon..
Normally the aperture stays open at the widest setting. You need as much light as possible to see your subject.
There is a small tab that moves in a slot on the back of the lens that will stop the lens down to the manually set aperture.

Set the aperture to say F/8. Then on the back, you can very very gently move the tab with your fingernail and the aperture blades will close.
It will spring back.
Then try the smallest aperture eg f/22

If the lens is in A mode, then push the round button down on the aperture ring, and turn to the manual setting you want.

[hatsofe]

Thanks. The matter is ,that the diaphragm remains fully open even been set to manual. Any A lens for 35mm Pentax camera allows to change the aperture manually out in the air, and without touching the lever on the mounting surface of the lens IF the lens aperture ring is set on any given aperture value. That lever on the back of the lens is getting pushed by the camera corresponding lever when the aperture ring of the lens is set on "A" (automatic). The lens I got doesn't change the diaphragm opening even if the ring is set to manual (not "A"). But it's possible, that if the lens is attached to the camera, that little aperture control lever is getting cocked by the camera lever all the time thus allowing manual aperture setting.

[Pål Jensen]

Thanks. The matter is ,that the diaphragm remains fully open even been set to manual. Any A lens for 35mm Pentax camera allows to change the aperture manually out in the air, and without touching the lever on the mounting surface of the lens IF the lens aperture ring is set on any given aperture value. That lever on the back of the lens is getting pushed by the camera corresponding lever when the aperture ring of the lens is set on "A" (automatic). The lens I got doesn't change the diaphragm opening even if the ring is set to manual (not "A"). But it's possible, that if the lens is attached to the camera, that little aperture control lever is getting cocked by the camera lever all the time thus allowing manual aperture setting.

It is normal for 645 lenses. It works the opposite way to K-mount lenses. The aperture remain open regardless when off camera.You can make the lens stop down by actuating the lever on the lens mount as the professor have already suggested.

[hatsofe]

It is normal for 645 lenses. It works the opposite way to K-mount lenses. The aperture remain open regardless when off camera.You can make the lens stop down by actuating the lever on the lens mount as the professor have already suggested.

I see. It's a relief. Thank you!