“Four-thirds is a measurement standard used to describe the sizes of ‘Vidicon’ tubes first used in video cameras. This style of measurement is now used to describe many of the CCDs used in digital cameras, like 1/2, 2/3, and now 4/3. Using this measurement, for example, a 1-inch tube (24.5 mm) will have an imaging area with a diagonal of about 16mm. Using this formula, a chip with a diagonal measurement of about 22.3mm would be a 4/3-type image sensor.
“Explaining their four-thirds choice, Olympus research found that the Four Thirds concept offered the best size-to-performance benefits of any format. Standardizing on this size allows Olympus to develop a total digital-specific system with digital-specific lenses, flashes, and bodies. By contrast, APS and 35mm-size sensors would require larger lens mounts and lenses. For example, on a 35mm, the lens mount would need to be about the size of one required for a medium format camera, which would make the 35mm a less usable field camera for professionals.”