When you install the lens, the full- open-metering post comes against the maximum-aperture correction pin, Fig. 12. In the Canon EF, pushing in the maximum-aperture correction pin moves the finder's diaphragm scale. But that's not the case with the AE-1-the diaphragm scale never moves. Instead, the maximum-aperture correction pin in the AE-1 controls a pair of variable resistors on the side of the mirror cage. One of the variable resistors provides information to both the aperture-control and readout systems. The other provides information to the readout (meter) only.