How is m4/3 different than 4/3?

m4/3 is mirrorless, while 4/3 is not. The (mostly) older 4/3 cameras were designed as DSLRs, complete with mirror box and optical viewfinders. Because of the mirror, the lens mount on the 4/3 cameras is far forward from the sensor. Because of the small sensor and small mirror, the viewfinders tended to be much like looking down a tunnel. On m4/3 cameras, the lens mount is very near the sensor. The viewfinders are magnified LCDs. While m4/3 and 4/3 share the same sensor size, they do not share the same lenses.

That said, you can mount 4/3 lenses via adapter on most m4/3 cameras. However, the only one that seems to have really good autofocus performance when you do is the Olympus E-M1 and E-M1m2. 

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