I just bought a used Bronica SQ-A which I have not even run the first roll of film through yet. It has an ME Prism Finder S. I can't figure out how to use it. The limited info I found on the net says there is a red LED that indicates over or under exposure and a green one that indicates correct exposure. I have played around with various shutter speeds and f-stops with no luck. Does there need to be film loaded? Help. Steve S.