Four Thirds system
The Four Thirds System is a standard created by Olympus and Kodak for digital SLR camera design and development. (Four Thirds is a registered trademark of Olympus Optical Co., Ltd.)
The system provides a standard which, with digital cameras and lenses available from multiple manufacturers, allows for the interchange of lenses and bodies from different manufacturers. U.S. patent 6,910,814 seems to cover the standard. Four Thirds is not an open standard, however, as it does not meet the "allowing anyone to use" criteria commonly accepted as the definition of an open standard. It also does not meet the criteria that the standard itself and any associated intellectual property be available on a Reasonable And Non-Discriminatory basis.
Unlike older SLR systems, Four Thirds has been designed from the ground up to be entirely digital. Lens design has been tailored to the requirements of digital sensors, most notably through telecentric designs. The advantages of the system include more compact telephoto lenses (a Four Thirds lens with a 300 mm focal length would cover about the same angle as a 600 mm focal length lens for the 35 mm film standard) and more even delivery of light to different parts of the sensor.
Sensor size and aspect ratio
The name of the system comes from the 4:3 aspect ratio of the sensor used in current cameras. The long side of the rectangular photographs that Four Thirds cameras produce is 4/3 the size of the short side. This is in contrast to the traditional 35 mm format with an aspect ratio of 3:2. The Four Thirds standard specifies the aspect ratio, in addition to the size of the imaging circle. Claim 1 of US patent 6,910,814, mentioned earlier in this article, is quite specific: "…said camera body having an image pickup device having an imaging range with an aspect ratio of 4:3 on an imaging surface within the image circle…"
A major reason for choosing 4:3 for the sensor proportions is that it has historically been the standard for television and became a dominant aspect ratio for computer monitors, as found in the VGA, SVGA, XGA, SXGA+, UXGA and QXGA standards.
It is necessary to know the aspect ratio in order to design lenses. Indeed, many lenses designed by Olympus for the four thirds system contain internal rectangular baffles or permanently mounted "petal" lens hoods that optimise the lenses operation for the 4:3 aspect ratio.
The size of the sensor is 18mm × 13.5mm (22.5mm diagonal). The imaging area is 17.3mm x 13.0mm (21.6mm diagonal). This is smaller than most other Digital SLR sensors.
Advantages and Disadvantages
Advantages
- Smaller sensor design is claimed to allow for smaller, lighter lenses with smaller optics, potentially allowing design of very small lenses (as Olympus did in the film OM-1 cameras), or very fast lenses at reasonable sizes, or very high quality lenses at reasonable cost. Currently this is evident to some extent in longer telephoto lenses.
- Telecentric optical path means that light hitting the sensor is traveling perpendicular to the sensor, resulting in brighter corners, particularly on wide angle lenses.
Disadvantages
- Smaller sensors are generally claimed to be more prone to noise.
- Smaller sensor means that a lens with the same effective focal length as a 35mm camera will result in twice the depth of field in the image. This is an advantage for landscape and macro photography, but seen to be a disadvantage when seeking to isolate a subject from a background.
Four Thirds system companies
As of the 2006 Photo Marketing Association Annual Convention and Trade Show, the Four Thirds consortium consists the following companies (in alphabetical order):
Four Thirds cameras
- Olympus E-1 digital camera (November 2003)
- Olympus E-300 digital camera (December 2004)
- Olympus E-500 digital camera (September 2005)
- Olympus E-330 digital camera (January 2006)
- Olympus E-400 digital camera (September 2006)
- Panasonic Lumix DMC-L1 camera (February 2006)
- Leica Digilux 3 camera (September 2006)
- Olympus E-P1 professional digital camera (No official release date - expected 2007)
- Olympus E-410 digital camera (April 2007)
- Olympus E-510 semi-professional digital camera (June 2007)
Four Thirds lenses
- Olympus has made 15 lenses for the Four Thirds System, with focal length ranging from 7 to 300mm, including macros
- Sigma has made or adapted 11 lenses for the Four Thirds System
- Leica has made 2 lenses for the Four Thirds System, a standard zoom with image stabilisation and a fast 25mm normal lens