On Mamiya M645 (the later ones) metered prisms, there is a switch to go from centered metering, average metering, and auto-selection. In auto-selection mode, depending on scene, the metering switches from center metering to average metering. Does anyone know what determines one mode to be selected over the other? I'm pretty sure this isn't a sophisticated system like Nikon's matrix metering or anything like it. But I'd like to understand what makes this system 'tick'.