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Four Thirds

In DSLR terminology Four Thirds refers to a standard that has been developed by Olympus and Kodak for Digital SLR cameras, the system ensures that various lenses and camera bodies can be interchanged freely without any compatibility issues. It has been developed as an open standard and many other camera and lens manufacturers such as Leica, Sigma, Fuji, Panasonic and Sanyo have began to produce products supporting the Four Thirds standard. As camera users and buyers such a standard can only make things easier for us, however we must still be aware that there are still many new products being released by major manufacturers that do not conform to the Four Thirds standard. Unfortunately the exacty details of Four Thirds are only available to camera manufacturers after they have signed a strict non disclosure agreement however the following information has been gleaned from various sources to help you understand exactly what Four Thirds is and how it works.

The name "Four Thirds" as I'm sure most of you will recognise is derived from the type of digital camera sensor that we are familar with, referred to as a 4/3 type sensor. The name is deliberately ironic as digital sensors are in fact usually 22.3mm, not 33.87mm (4/3 of an inch) as most would assume, the 4/3 of an inch measurement coming from a time when vacuum tubes were used as image sensing devices and were measured in a different way. In a convulution of circumstances this actually sums up the Four Thirds standard rather well; breaking away from the past and embracing technology to push DSLR photography past what was possible with conventional wet film cameras and their predecessors.

Four Thirds is based soley around digital SLR photography, no wet film cameras are included in the standard. One of the impacts of Four Thirds is that a large number of lenses have been created just for use with digital SLR cameras, these lenses are fully optimised to take advantage of digital sensors without loosing any field of view as you would if you chose to use many conventional 35mm lenses with digital cameras. Olympus explain that "the standard focal lens of the Four Thirds standard is around 25mm. So to compare for the 35mm format with a standard focal length of around 50mm, you have to use the factor 2 to have the angle of view compared (50mm = 100mm related to 35mm equivalent focal length)." To summarise, the angle you see through the viewfinder you physically see when composing your shot is exactly what your camera will produce as a picture.

Due to the new technology being used in the manufacture of camera lenses it is now possible to grind lenses that are far smoother and clearer than ever before. The oft quoted figure of 10 microns appears to be the resolution produced by conventional cameras, however Four Thirds lenses are manufacturered with digital sensors in mind and can produce resolutions with a pixel pitch down to and including 5 microns (or even less in some of the more expensive Four Thirds compatible lenses) which allows us to take full advantage of the high levels of detail offered by modern digital sensors. The physical shape and construction of the lens also helps maintain a high standard of image quality. Due to the nature of how an image sensor operates there is often a loss of sharpness towards the edge of an image, this is due to light hitting the pixels of the sensor at a very high angle of incidence (wet film is far more forgiving with this phenomenon than digital image sensors). The telecentric construction of all Four Thirds standard lenses ensures that light passing through the lens hits the the sensor with as low an angle of incidence as possible, usually at right angles to it resulting in no loss of picture quality at the edges of an image.

You can find more information about Four Thirds at official website.