Sensor Size and Aspect Ratio
The image sensor of Four Thirds and MFT is commonly referred to as a 4/3" type or 4/3 type sensor (inch-based sizing system is derived from now obsolete video camera tubes). The sensor measures 18 mm × 13.5 mm (22.5 mm diagonal), with an imaging area of 17.3 mm × 13.0 mm (21.6 mm diagonal), comparable to the frame size of 110 film. Its area, ca. 220 mm², is approximately 40% less than the APS-C sensors used in other manufacturers' DSLRs, yet is around 9 times larger than the 1/2.5" sensors typically used in compact digital cameras.
The Four Thirds system uses a 4:3 image aspect ratio, in common with other compact digital cameras but unlike APS-C or full-frame DSLRs which usually adhere to the 3:2 aspect ratio of the traditional 35 mm format. Thus "The Four Thirds refers to both the size of the imager and the aspect ratio of the sensor". Note that actual size of the chip is considerably less than 4/3 of an inch, the length of the diagonal being only 22.5 mm. The 4/3 inch designation for this size of sensor dates back to the 1950s and vidicon tubes, when the external diameter of the camera tube was measured, not the active area.
The MFT design standard also calls for being able to record multiple formats, 4:3, 3:2 (traditional DSLR formats which have origins with 35 mm film still cameras), 16:9 (the native HD video format specification), and 1:1 (a square format). With the exception of two MFT cameras, all MFT cameras record in a native 4:3 format image aspect ratio, and through cropping of the 4:3 image, can record in 16:9, 3:2 and 1:1 formats. This multiple recording format flexibility is a MFT system design standard, which also incorporates seamless integration of HD video recording in the same camera body.
The 2009 introduction of the Panasonic Lumix DMC-GH1 camera extends the 4:3 format image aspect ratio recording capabilities to native 16:9 and 3:2 image aspect ratio formats, rather than crops of a native 4:3 image. The GH1 uses a bigger sensor matrix that uses the full diagonal of the image circle in all three formats. This is called multi-aspect capability. To date, the multi-aspect sensor is common only to the Panasonic GH1 and its successor the Panasonic Lumix DMC-GH2.
In addition, all current Micro Four Thirds cameras have sensor dust removal technologies, but this is not exclusive to the format.
Read more about this topic: Micro Four Thirds System
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