Earlier section mentioned and illustrated Body and lens coupling was based on a newer FDn lens and here is an illustration which used an original older version of an FD lens to demonstrate. This is the conventional stopped-down coupling lever which automatically resets to full aperture opening position after shutter release. Full-Aperture Signal Pin . In order to correct this error, the full-aperture signal pin plays a role to adjust the matching needle position. Aperture Signal Lever.
First function is the aperture transmission signal in full-aperture metering.
At this time, of course, light metering is full-aperture method. When the preset aperture ring is set at the 'A' mark, located outside the scale, this lever is automatically disconnected from the preset aperture ring and can be freely moved. This function is made possible in EE photography with the EF and Servo EE Finder attached to the F-1. In other words, for other older Canon bodies, this lever is turned to the proper position for closing down the aperture upon information received from the EF and Servo EE Finder. Thus the proper exposure can be obtained and unmanned EE photography is possible.