Recently I purchased a tamron 105/2.5 adaptamatic(I think) lens with an M42 mount, which is shown in the picture below. I could mount it on my k-x with the original pentax screw mount adapter and use it in M mode with the green button metering. But to stop down the lens after setting the aperture on the aperture ring I have to 'hold' the A/M switch at A position. If I leave the switch it comes back to the M position and the lens becomes wide open. The only other M42 lenses I have seen are Super/SMC takumars and there the A/M switch remains at whichever position I keep.
Unlike in the case of takumars, here the 'pin' that moves the aperture blades does not stay depressed when the lens is mounted. But once I press the pin the lens stops down properly even if the A/M switch is at M. So one possible fix could be to glue the pin at its depressed state.
I wonder if this is the expected behavior of this lens or if something is wrong with the A/M switch. Or do I have to use any other adapter to make it work?
Please help me out.
Note to moderators - You may move this thread to a more appropriate section if needed.