Sorry to post this in multiple forums but I'm new and realized this post may belong here instead. A friend just gave me an old (1985) Sigma Zoom-Master 35-70mm OM mount lens. When I depress the depth of field preview button on the side, I can't see the aperture stop down inside the lens (I've set it at its minimum, f/22). By means of comparison, I can see the aperture stop down on the Zuiko 50mm I have. So the question is: is it broken? I don't want to waste time or film to find out (e.g., shooting a roll of film, developing it, and learning that yes, the aperture really is stuck wide open). Thanks.