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