If you hold the Depth of Field preview button (the button opposite the unlock button on the lens), that will manually stop down the aperture. Meaning, you are not just stuck at wide-open aperture. The only minor caveat is that you must hold this button down while you snap the photo. For some reason, the manufacturers of this adapter chose to include a stop that prevents the slider on the back of the lens from moving and thus stopping down the lens. This makes no sense to me why they'd go out of their way to prevent this very useful function.

Even more confusing is why it's so easily removed. You just need some precision screwdrivers and half a brain. Remove the four screws on the silver plate, lift the plate off the adapter, and then you will find one small screw holding the stop in place. Unscrew the screw and remove the stop. Then just reassemble the back plate with the orientation shown in the image (the notched blade is closest to where the text "OM" is printed).

You should be good to go! Now, if you hold the DoF preview button down while snapping your picture, the lens will stop down to whatever aperture you have the ring set to and you will be able to use the lens to its fullest potential. In auto mode, the camera will automatically adjust exposure once the aperture is stopped down.