Patent Application 20100259666 from Sony is about a transmissive liquid crystal panel that could be implemented in the optical finder and can provide superimposed indication of various parameters, such as AF area:
The patent includes also diagrams of what appears to be a Sony camera body:
From the patent application:
"Digital single-lens reflex cameras and other imaging apparatuses often shift to a state in which when the power source is turned off by a power switch or a switchover to a power saving mode is made, immediate photographing is not allowed (photography is prohibited) even when a shutter button is operated. Since this photography prohibited state prevents immediate photographing from being carried out even when a user tries to take a photograph by looking into a finder window, photo opportunities are sometimes missed.
Imaging apparatuses are proposed that are provided with a transmissive liquid crystal panel on a light path of an optical finder and can carry out a superimposed indication in which indications, such as a focus detection region (AF area), are superimposed on a photographic subject image in the finder window upon taking a photograph by varying the transmittance."
