Sensor Size and Aspect Ratio
The name of the system stems from the size of the image sensor used in the cameras, which is commonly referred to as a 4/3" type or 4/3 type sensor. The common inch-based sizing system is derived from vacuum image-sensing video camera tubes, which are now obsolete. The imaging area of a Four Thirds sensor is equal to that of a video camera tube of 4/3" diameter.
The usual size of the sensor is 18×13.5 mm (22.5 mm diagonal), with an imaging area of 17.3×13.0 mm (21.63 mm diagonal). The sensor's area is about 30–40% smaller than APS-C sensors used in most other DSLRs, but still around 9 times larger than the 1/2.5" sensors typically used in compact digital cameras. Incidentally, the imaging area of a Four Thirds sensor is almost identical to that of 110 film.
The emphasis on the 4:3 image aspect ratio sets Four Thirds apart from other DSLR systems, which usually adhere to the 3:2 aspect ratio of the traditional 35 mm format. However, the standard only specifies the sensor diagonal, thus Four Thirds cameras using the standard 3:2 aspect ratio would be possible; notably newer Panasonic Micro Four Thirds models even offer shooting at multiple aspect ratios while maintaining the same image diagonal. For instance, the Panasonic GH1 uses a multi-aspect sensor designed to maximize use of the image circle at 4:3, 3:2, and 16:9; each ratio having a diagonal of 22.5mm.
Sensor aspect ratio has an impact on lens design. For example, many lenses designed by Olympus for the Four Thirds system contain internal rectangular baffles or permanently mounted "petal" lens hoods that optimise their operation for the 4:3 aspect ratio.
In an interview John Knaur, a Senior Product Manager at Olympus, stated that "The FourThirds refers to both the size of the imager and the aspect ratio of the sensor". He also pointed out the similarities between 4:3 and the standard printing size of 8×10 as well as medium format 6×4.5 and 6×7 cameras, thus helping explain Olympus' rationale on choosing 4:3 rather than 3:2.
Read more about this topic: Four Thirds System
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