I have acquired a Vivitar 3500 and would appreciate some advice from an owner as to its use. It should be simple but I am confused by the two separate controls on which the ISO is set. I am assuming that only one of them is a switch and the other is just an indicator. Mine has the Nikon module and the lower switch has a position marked FG. Does this refer to the Nikon FG? I'm hoping to use it with a Nikon FE2. Thanks, Steve.