. The original Four-Thirds system is based on a 1/4 frame (18mm x 13.5mm) digital sensor (with a 2x lens multiplier factor). The promise of smaller, lighter, cheaper DSLRs was one of the initial selling features of the system, though that really hasn't proven to be the case. The difference in size weight and cost between the Olympus Four-Thirds DSLRs and the similar APS-C offerings from other manufacturers isn't really very significant.
However the proposed micro Four-Thirds system takes the size of cameras (and perhaps also lenses), down another notch. We don't know anything and pricing yet of course.
The basic principle behind the micro Four-Thirds system is the elimination of the reflex viewing mirror (that's the "R" in SLR) and associated components such as the mirror box, pentaprism and optical viewfinder. This allows a much thinner camera body to be designed and also allows the lens to be much closer to the digital sensor. Being closer to the digital sensor has some optical advantages for wideangle lenses because it allows short focal length lenses to be used without requiring extreme retrofocus designs.
Though the flange to sensor distance of the micro Four-Thirds system is 20mm, since there is no reflex mirror to avoid, the rear element of wideangle lenses could project a significant distance into the camera. This would be similar to the way some wideangle lenses project inside the body of Leica rangefinders cameras (though the Four-Thirds sensor is only 1/4 the size of 35mm film).
Another advantage of the very short 20mm flange to sensor distance is the fact that all lenses designed for 35mm use have a longer back focus distance than this, and so in principle could be adapted for use on a micro Four-Thirds body without requiring any additional optics for infinity focus. Modern AF lenses of other brands (Nikon, Canon etc.) would not be electronically compatible and so would not autofocus or have any electronic aperture control and so wouldn't be very compatible. However manual focus lenses in just about every mount from Canon FD to Pentax-K should be usable if the right adapters were
available.
