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What does the R/S function do internally and should it be in the R position for roll film. Thanks

The function of the roll/sheet selector is only to disable the shutter interlock preventing double exposures with roll film when you use sheet film. As it is not convenient to use sheet film with the G690 camera (one cannot load the film in daylight without a light-tight changing bag), this selector is rarely used. Nevertheless, this feature can also be used to cock the shutter without advancing the film, like for multi-exposure pictures, or when you mount a lens with an uncocked shutter on a body with the film already wound.

Cheers,

Sebastien