Meter Coupling

Just where the lens would mount to the camera body, on the aperture ring you see the metering coupling ridge engaged to the camera body's meter tab that allows the internal meter in the camera body to 'see' where the lens aperture ring is set to allow the diaphragm to remain 'wide open' for composing the shot.  You can also see the release button that allows you to retract this tab to use older series Nikkor F-Mount lenses on this model camera f16crewchief.deviantart.com/ar… .

The metering tabs position pretty much lines up with the older meter coupling prongs of the Nikon-F, F2 and Nikkormat bodies.