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Four Thirds is a system pioneered by Olympus and Kodak and designed exclusively for digital SLR cameras. It exists as an open standard and is supported by a consortium of manufacturers including: Olympus, Sigma, Panasonic, Leica, Sanyo and Fujifilm.

The Four Thirds Standard Defined

As a mechanical standard, Four Thirds uses a sensor size roughly half that of 35mm film. It also uses a specific type and size of lens mount and flange. It also has its own image circle size and communication standard for information exchange. What this means is that only Four Thirds lenses can be used on Four Thirds SLR cameras such as Olympus. The benefit of it as an open standard, is that rather than each of the participating manufacturers having to develop their own set of lenses to compete against the magnitude and variety of industry leaders Canon and Nikon, any Four Thirds lens, be it is Sigma, Leica, Panasonic or Olympus, will work on any Four Thirds camera. Remember that Sigma and Leica also produce lenses for other mounts, so it must be a Four Thirds mount lens.

Four Thirds Benefits

One of the goals in the development of the Four Thirds system was to produce cameras and lenses that find a balance between compact and lightweight design and image quality. While the sensor size is smaller than 35mm full frame and APS-C size sensors, there are benefits that derive from this in image quality and compact design.

From a design point of view, having a smaller sensor about half the size of a 35mm full frame sensor means that the camera effectively has a 2x crop factor when it comes to focal length. So a 300mm lens in Four Thirds is a 35mm equivalent focal length of 600mm. APS-C size sensors have a crop factor of between 1.5 and 1.6 times. So 300mm lenses in APS-C cameras have 35mm equivalent focal lengths of between 450mm and 480mm. This means that to achieve the equivalent perspective on a 35mm camera, a Four Thirds camera requires a lens with half the focal length. Effectively, a far smaller and lighter lens. The smaller sensor size also allows the camera bodies themselves to be made more compact. Four Thirds cameras lead the DSLR product category in lightweight and compact design.

From a picture quality point of view, the smaller sensor size and hence smaller focal length needed, allows flexibility in lens design to produce either faster lenses (brighter lenses with wider lower f-stops) or far more compact lenses. This is because a lens’s f-stop is determined by the ratio of the focal length to the diameter of the aperture. As can be seen from the table below, a lens diameter half the size of its 35mm counterpart is required to produce the same maximum f-stop. Alternatively, a lens can be constructed similar in size to a 35mm lens that is 2 stops brighter (150mm divided by 75mm = f2). This allows Four Thirds designers to create lenses that have powerful telephoto reaches and maximum apertures that match their 35mm counterparts, while still being smaller and lighter. It also allows them to construct pro lenses that are far faster than their 35mm counterparts.

As a dedicated digital system, all Four Thirds lenses also have higher resolutions than their 35mm counterparts to compensate for the smaller sensor size of the system, allowing them to resolve the same amount of detail.

Light striking an APS-C size sensor

Light striking a Four Thirds size sensor

The second and very important characteristic of the Four Thirds system is the telecentric lens to image sensor design. Unlike film, the pixel sites on a digital sensor optimally require light to strike the sensor surface perpendicularly. With larger format systems, the image sensor is larger relative to the size of the lens mount. This makes it more difficult for the lens to bend the light to reach the pixel site perpendicularly, especially around the periphery of the sensor. This is particularly evident in wide angle lenses where the angle of light entering the lens is more oblique than in telephoto lenses. To some extent this was improved by the introduction of microlenses on the pixel sites of digital sensors, but the degree of correction is best on the telecentric design of the Four Thirds system. Non-telecentric designs are susceptible to such phenomena as corner shading – where the periphery of the picture is slightly darker than the centre of the picture; loss of sharpness, ghosting and flaring; and chromatic abberation.

In the Four Thirds system the lens mount is about twice the size of the image circle. Allowing light to be bent to strike the sensor surface at a near perpendicular angle, which has effectively resolved issues of corner shading and improved related sharpness, aberration and flaring issues.

Drawbacks of Four Thirds

Both an advantage and a disadvantage is the greater depth of field available at any given aperture over an APS-C or Full Frame system. This is because the smaller image sensor size has a smaller physical aperture size (refer to the Aperture size comparisons table above). This is advantageous for landscapes and macro photography, but undesirable for portraits and selective focus applications. To compensate for this, there are faster lenses available that allow for the use of smaller f-stops.

A second drawback is that the image sensor size will ultimately limit the megapixel resolution these cameras can achieve. At present they comfortably cope with 12 megapixel resolutions and can theoretically be extended up to 25 megapixels. There is a price to be paid for higher megapixel resolutions, in the necessity for great lens resolution and the possibility of introducing more noise. Megapixels ultimately relate to intended print size and unless the user intends to print pictures to be used as billboards down the sides of buildings, it is not necessary to use anything higher than 10 megapixels. The megapixel race in the consumer market is largely a marketing issue, as manufacturers are aware that consumers use this as a primary quality gauge.