Quote Originally Posted by E. von Hoegh View Post
Chan, I think pushing the pin on the lens doesn't close the diaphragm, it releases the diaphragm so it can be closed by an internal spring. This is why it doesn't feel tight. Even if it doesn't work the way I think, it would take very little drag to slow things down. When the lens is pointing up, the blades are probably lying on each other and dragging.

Check it with a different lens on that body to be certain it is the lens.
This is the screw mount lens exactly it's the Auto Takuma 55mm f/1.8. Pushing the pin IN close the aperture when the stop down switch on the lens is set at A. When it's set at M the lens is always stopped down to the set aperture. Not like the new Pentax K mount that letting go of the level close the aperture.