While the major players in the DSLR camera market based their digital cameras on their existing film camera mounts and lenses, Olympus did not have a significant user base at this time, and found it better to design a new system from ground up, based on the requirements of digital photography.
Aspect ratio and sensor size
The name "Four Thirds" cleverly refers to two things at the same time:
- The image aspect ratio. While the aspect ratio of 35mm film cameras is 3:2, the Four Thirds system has a more square aspect ratio of 4:3. There are many reasons for this, e.g., a typical print size is 8:10, with a more similar aspect ratio to 4:3. And professional medium formats are typically closer to square aspects, like 4.5x6, 6x6 and 6x7. However, since the introduction of Four Thirds, formats that are even wider than 3:2 have become popular. TV sets are now typically 16:9. I still think that 4:3 makes sense for photography. A more square format uses the image circle more efficiently, which is an advantage, assuming, of course, that you're not going to crop it down to a more rectangular format.
- The size of the sensor. The sensor has the same size as the imageing area of a 4/3'' diameter legacy cathode ray tube for video cameras. The actual size of the sensor is still significantly smaller than 4/3'', with the diagonal of the imaging area usually being 21.6 mm.
- In theory, a smaller sensor usually means worse high ISO capabilities, and worse dynamic range characteristics, given the same number of pixels, and the same sensor technology.
- The depth of field is larger with a smaller sensor. This means that given the same distance, field of view and aperture, more will be in focus with a smaller sensor. This is both an advantage and a disadvantage. In landscape photography, you will most often want sharpness across the image, and a wide depth of field. On the other hand, in portrait photography, it is often desireable to have a narrow depth of field, to achieve isolation of the subject, and blurring of the background.
- A smaller sensor will limit the useful range of apertures more, due to diffraction, again assuming the same number of pixels. Diffraction will make the image blurry at pixel level when using small apertures. This is not due to bad design, but rather the result of the laws of nature.
- Designing lenses and cameras around smaller sensors is in some ways easier: They can be made more compact.
- A small sensor gives a higher crop factor. The crop factor of the Four Thirds sensor is roughly 2x, meaning that the focal length in mm for a Four Thirds lens must be multiplied with 2 to get the equivalent focal length on a traditional 35mm camera. Hence, a 14mm lens on Four Thirds is a wide angle lens, corresponding to 28mm on a traditional film camera. Some suggest that with this background, systems with a high crop factor are good for people who need long lenses, e.g., for wildlife photography. However, there are lenses to suit virtually all needs in the Four Thirds system.
Another important aspect of the Four Thirds mount is the register distance, also called the flange focal distance. The register distance is the distance between the lens mount and the sensor plane. With the sensor size being smaller than for other DSLR systems, the Four Thirds system also has a smaller register distance. This aspect of the camera systems makes designing lenses, especially wide angle lenses, easier.
Olympus lenses are commonly more telecentric than lenses from other systems, especially lenses that were originally designed for film. The telecentric property of a lens means that the light rays coming from the lens hit the sensor at a more normal angle across the field, especially in the corners. This is important for digital photography, since the sensors are sensitive to the angle of the light rays: If the light rays are hitting the sensor at a narrower angle in the corners, a significant portion of the light will be lost in the sensor pixel bins, giving heavy vignetting. More telecentric lenses generally give less vignetting in the corners. However, designing telecentric lenses is more complicated, and some of the size advantage of the smaller sensor is lost with this property of the lenses.
The original cameras in the Four Thirds system were not so compact, and, while they were innovative, they were also somewhat strange looking. Later, Olympus launched smaller cameras, with the E-4xx series being the smallest DSLRs on the market at the time of availability. The cameras have also been more traditional looking after the first two-three years. After some years of maturing, the Olympus DSLRs are not very different from the competitors, beyond being somewhat smaller.
Panasonic and Leica have also issued cameras in the Four Thirds system, however with major components Olympus sourced. The Leica branded cameras have largely been similar to Panasonic models, with smaller changes to the design and software. Since this time, both Panasonic and Leica appear to have withdrawn from the Four Thirds system, with Panasonic focusing more on the newer and more compact Micro Four Thirds system, and Leica designing their own high end cameras, e.g., the Leica X1. Leica still appears to have some involvement in the design of lenses for the Micro Four Thirds system, with the Leica branded 45mm f/2.8 macro lens launched in the autumn 2009.
A new standard was announced in 2008, the Micro Four Thirds. This shares the sensor size with Four Thirds, however, leaving no room for a mirror assembly. Panasonic G1, the first camera for this standard, was launched in the autumn 2008. The new system has been very successful in Japan, and Panasonic cameras and lenses have been scarce in Europe and especially the US in 2009, suggesting a high demand.
According to the statistics quoted above, the Olympus E-P1 Micro Four Thirds camera was the most selling Olympus camera model in Japan 2009. This has prompted some concern that Olympus might discontinue the regular Four Thirds line, and focus exclusively on Micro Four Thirds. I think the chances for this are slim at the moment, since Olympus has a large and good back catalogue of Four Thirds lenses. While the E-P1 camera has sold a lot, I am sure that they have sold a lot of Four Thirds lenses as well, not recorded by the statistics.