If you unscrew the front elements there is a retaining ring that holds the aperture ring in place. Slip it back into the groove it is supposed to ride in and it will be fixed. Careful that you don't lose the ball bearing while doing so. Keep pressure on the aperture ring after you remove the front elements and don't turn the lens upside down or remove the aperture ring. One of mine does this occasionally. I takes only a minute to fix.
Originally Posted by Heywoodphoto
Nearly every Computar I have seen exhibits some separation in the rear elements. Not really worth fixing either due to the cost of it. They are great lenses though. I have three.