There are two levers on the back of the lens. The one opposite the infinity mark stops the lens down and the one at the side controls how far it can stop down.
The problem's the body not the lens.
To check the lens, set the lock as though it was mounted on the body and move it to the right. The lens should stop down. Next would be to move the lever (or change the f stop ring)at the side and notice the aperture. As you move the lever to different positions the aperture should stop when it reaches it's limit.
If it's working manually, The lever on the side of the mirror box isn't working properly(no duh!) That's the lever that controls aperture in auto. It limits where the aperture will stop.
Try holding the lever up as you trip the camera and it should want to move downward. in auto the lever on the lens would move to whatever the camera calculates is proper and holds it. If it's not working, the lever in the body doesn't move at all keeping the lens wide open. This has no effect in manual..........So, use it manually, be happy.
That was my guess at this point as well. Everything works in manual mode (aperture, FOV button, meter, etc...). Was hoping it would work in auto mode just for quick shots, but it's no biggie other than the inconvenience of having to take the viewfinder away from my eye to check the aperture after metering. That's one thing I like about my Pentax MX is that you can see what f-stop you're on in the viewfinder.
Originally Posted by John Koehrer
Here's another shot of a 50mm f1.8 FD lens that came with the camera I found on the net. It's markings are exactly like mine.