how does it work ?

Are you telling us that the translucent mirror is only used for measuring distance and light value, while the EVF image is actually produced by the APC-S sensor ? That makes sense. However, if so, the A55, and her SLT alikes, would have to be classified as "DSLR-likes" and not as mirror reflex types.

Reflex cameras - I think we all agree on that - produce the viewer's image (OVF and EVF ....) via the mirror.

Chimere