Question about the Canon EX Auto

The Canon EX Auto is an SLR that does not use traditional FL/FD lenses but, instead, uses different screw-on front element sets in order to achieve different focal lengths. When you unscrew the front element you see the rear element set just under the aperture mechanism.

My question is this: inside the rigid mount of the camera, just under the threads used for mounting the front element set, there is a tiny button that gets pressed down when the front element set is screwed on. What does that button do? Thanks. - David Lyga

I believe it's the signal pin that communicates the maximum aperture of the interchangeable lens component to the camera's meter. The shoulders of the 35mm, 95mm and 125mm components (behind the lens mount threads) push the pin further into the lens than the 50mm (signaling f/3.5 vs f1.8 maximum apertures). Canon's FD lenses have maximum aperture signal pins that perform a similar function with FD bodies.

Oh, I see. Each of the various front element sets has a different amount of extension onto that pin. Thank you. - David Lyga