Lens Aperture Question?

I have a SMC Pentax-M 50mm lens which I have never used. When turning the aperture ring off-camera the aperture opens and closes as normal. However, when in the process of mounting the lens the aperture opens fully and stays that way. Even when fully mounted, operating the aperture ring has no visible effect.
Can any of you lettered men of science explain this?
Is the camera or lens faulty?
Is it me?

All lenses open up to full aperture when you mount them on the camera.

That way, you can see to focus and frame your shot.

When you press the shutter, the camera stops the lens down to the aperture you have selected.

Note that when using non-A lenses (like your M), the camera [if it is a DSLR] will only stop down to the set aperture if the exposure mode is set to manual. Otherwise, it will fire at full aperture.

G

Thanks very much, George.