Gene Wright talks about the Four Thirds System

The Four Thirds Specification defines the standard diagonal length of the 4/3-type image sensor, suitable image circle of lens and an interface between lens and body. This will make it possible to secure compatibility among camera bodies and lenses regardless of manufacturer or model.

The Four Thirds system is a standard created by Olympus and Kodak for digital single-lens reflex camera (DSLR) design and development.

"Four Thirds System. Different from older SLR and DSLR systems, Four Thirds was designed from the base up to be totally digital. Many of the lenses have been significantly computerized, to a point that companies are providing firmware updates for a large number of them. Lens designs have been adapted to digital sensor requirements, most particularly through telecentric designing. The sensor size is somewhat less than a majority of DSLR cameras which implies that lenses, more than ever a telephoto lens, can be made quite a lot smaller. As an example, a Four Thirds System lens featuring a a focal length of 300mm could cover virtually the same viewing angle as a 600mm focal length lens as a 35 mm film lens, and is equally more compact. The Four Thirds System has a focal length multiplier (crop facto) of around 2. May 15, 2011"

Sensor size and aspect ratio

The size of the sensor is 18ื13.5 mm (22.5 mm diagonal), with an imaging area of 17.3ื13.0 mm (21.63 mm diagonal). Its area is 30–40% less than the nearly APS-C sensors used in most other DSLRs, but around 9 times larger than the 1/2.5" sensors typically used in Digital Camera Review by Gene Wrights (see image sensor format).

The Four Thirds system uses a 4:3 image aspect ratio, in common with Digital Camera Review by Gene Wrights. This differs from other DSLRs which usually adhere to the 3:2 aspect ratio of the traditional 35 mm format. 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) is 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 to choose 4:3 sensor proportions was similarity to the aspect ratio on standard definition television. Computer monitors also commonly use a 4:3 aspect ratio, as found in the VGA, SVGA, XGA, SXGA+, UXGA and QXGA standards.

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.

John Knaur, a Senior Product Manager at Olympus, states that "The FourThirds refers to both the size of the imager and the aspect ratio of the sensor". He goes on to state the similarity between 4:3 and the standard printing size of 8ื10, as well as medium format 6ื4.5 and 6ื7 cameras.

Advantages, disadvantages and other factors

Advantages

• The smaller sensor size makes possible smaller and lighter camera bodies and lenses. In particular, the potential exists for very fast lenses and very high quality lenses at lower costs. Currently this is evident to some extent in the Olympus E-4x0 and E-620 bodies, in the kit lenses sold with the E-4x0 and E-5x0 bodies, and in longer telephoto lenses.
• Greater depth of field at any given aperture and focal length due to the smaller format. This is an advantage in many applications (macrophotography in particular), but a disadvantage in others (portraiture) where a shallow depth of field is desired.
• Telecentric optical path means that light hitting the sensor is traveling perpendicular to the sensor, resulting in brighter corners, and most importantly improved off-center resolution, particularly on wide angle lenses.
• Because the flange focal distance is significantly shorter than most competing mounts (such as Canon FD, Canon EF, Nikon F and Pentax K), lenses for many other SLR types, including the old Olympus OM System, can be fitted to Four Thirds cameras with simple mechanical adapter rings. (Such mechanical adapter rings typically require manual setting of focus and aperture). In many cases this produces excellent results, especially with longer focal-length lenses and lenses at smaller apertures. A series of tests by John Foster (Using OM legacy lenses on E1 body) provides a demonstration.

Disadvantages

• Smaller sensors collect less light in total than larger ones, and As a result have a weaker signal-to-noise ratio. Images made with a Four Thirds sensor will show more noise at the same sensitivity than those made with larger formats, although the noise performance of Four Thirds is only slightly worse than APS-C.
• A telecentric optical path means more aggressive retrofocus design for wide and normal lenses, which makes them bigger, and makes wide apertures harder to achieve.

Other factors

Four Thirds system companies

• Fuji
• Kodak
• Leica
• Olympus
• Panasonic
• Sanyo
• Sigma

Four Thirds system cameras

Olympus' E-system camera bodies are noted for their inclusion of a wide range of firmware-level features and customization, good JPEG engine, and compact size. Because of the smaller format of Four Thirds, the viewfinders tend to be smaller than on comparable cameras. 

• Olympus E-1 (November 2003)
• Olympus E-300 (December 2004)
• Olympus E-500 (September 2005)
• Olympus E-330 (January 2006)
• Panasonic Lumix DMC-L1 (February 2006)
• Leica Digilux 3 (September 14, 2006)

Four lenses for the Four Thirds system. These are three Olympus zooms (40–150 mm, 11–22 mm and 14–54 mm) and a Sigma prime (30 mm).

• Olympus E-400 (September 2006)
• Olympus E-410 (April 2007)
• Olympus E-510 (June 2007)
• Panasonic Lumix DMC-L10 (October 2007)
• Olympus E-3 (November 2007)
• Olympus E-420 (May 2008)
• Olympus E-520 (June 2008)
• Panasonic Lumix G1 (Sep12, 2008)
• Olympus E-30 (November 2008)
• Olympus E-620 (March 2009)
• Olympus E-450 (April 2009)
• Panasonic Lumix DMC-GF1 (September 2009)
Olympus PEN E-P1 12.3 MP Micro Four Thirds Digital Camera - Jun 16, 2009
• Olympus PEN E-P2 12.3 MP Micro Four Thirds Digital Camera - Nov 5, 2009
• Olympus PEN E-PL1 12.3MP Live MOS Micro Four Thirds -
Feb 3, 2010
• Panasonic DMC-GH1 12.1MP Four Thirds Camera - Feb 26, 2010 - Announced Mar 3, 2009
• Panasonic Lumix DMC-GF1 12.1MP Micro Digital Camera - Oct 28, 2009

Four Thirds system lenses

There are currently around three dozen lenses for the Four Thirds system standard.

Olympus is producing about 20 lenses for the Four Thirds system under Zuiko Digital brand. These are divided into three "grades" (Standard, Pro, Top Pro). The higher grade lenses have faster maximum apertures but are significantly more expensive and larger, and the Top Pro grade zooms have constant maximum aperture over the full zoom range; all but the Standard grade are weather-sealed. Lenses within each grade cover the entire range from ultra-wide-angle to super-telephoto. The Zuiko Digital lenses are well-regarded by reviewers for their consistently good optics. The following is a table of all current Zuiko Digital lenses:

Olympus also makes 1.4ื and 2ื teleconverters and an electronically-coupled extension tube. Sigma has adapted 12 lenses for the Four Thirds system, ranging from 10 to 800 mm, including several for which no equivalent exists: the fast primes (30 mm f/1.4 and 50 mm f/1.4) and extreme telephoto (50–500 mm f/4–6.3). Leica has made four lenses for the Four Thirds system: fast and slow normal zooms and a 14–150 mm super-zoom, all with Panasonic's image stabilization system, and an unstabilized f/1.4 25 mm prime. An official list of available lenses can be found on Four-Thirds.org web site

Micro Four Thirds system

Concept Micro Four Thirds camera by OlympusIn August 2008, Olympus and Panasonic introduced a new format: Micro Four Thirds.

The new system uses the same sensor, but removes the mirror box from the camera design. A live view preview is shown on either the camera's main LCD or via an electronic viewfinder, as in digital compact cameras. Autofocus is accomplished via a contrast-detect process using the main imager, again similar to digital compact cameras. The goal of the new system is to allow for even smaller cameras, competing directly with higher-end point-and-shoot Digital Cameras and lower-end DSLRs. The smaller flange focal distance allows for more compact normal and wide-angle lenses.

Four Thirds lenses can be used on Micro Four Thirds bodies with an adapter. Many do not support contrast-detect autofocus and must be manually focused.

Micro Four Thirds system cameras

The first Micro Four Thirds system camera was Panasonic Lumix DMC-G1, which was commenced in Japan in October 2008. In April 2009, Panasonic Lumix DMC-GH1 with HD video recording added to it.

The first Olympus model, Olympus PEN E-P1 was shipped in July 2009.

• Panasonic Lumix DMC-G1 (October 2008)
• Panasonic Lumix DMC G2 (Mar 2010)
• Panasonic LUMIX DMC-G3 (May 2011)
• Panasonic Lumix DMC-GH1 (April 2009)
• Panasonic Lumix DMC-GH2 (December 2010)
• Panasonic Lumix DMC-GF1 (September 2009)
• Panasonic Lumix DMC GF2 (November 2010)
• Olympus PEN E-P1 (July 2009)
• Olympus PEN E-P2(November 2009)
• Olympus PEN E-PL1 (February 2010)
• Olympus PEN E-PL2

Micro Four Thirds lenses

Standard zooms

• Olympus mZD 14–42mm f/3.5–5.6 (E-P1 kit)
• Panasonic Lumix 14–45mm f/3.5–5.6 (G1/GF1 kit)

• Olympus mZD 14-150mm f/4.0-5.6
• Panasonic Lumix 14–140mm HD f/4–5.8 (GH1 kit) 

• Panasonic Lumix 45–200mm f/4–5.6 (G1 kit)

• Olympus mZD 9-18mm f/4.0-5.6 - May 15, 2011
• Panasonic Lumix 7–14mm f/4

• Olympus mZD 17mm f/2.8 (E-P1 kit)
• Panasonic Lumix 20mm f/1.7 (GF1 kit)
• Voigtlander Nokton 25mm f/0.95  

• Panasonic Leica DG Macro-Elmarit 45mm f/2.8 Asph.

Further, both Panasonic and Olympus manufacture an adapter to enable use of any Four Thirds lenses on Micro Four Thirds cameras. While many Four Thirds lenses accept firmware updates to enable contrast autofocusing, many others are manual-focus-only. A variety of companies manufacture adapters to use lenses from nearly any legacy lens mount[6] (such lenses, of course, support no automatic

Four Thirds and Micro Four Thirds Lens Adapters
Have an old lens you want to mount on your new Four Thirds System Camera? Now you can adapt Leica, Olympus, Nikon, Panasonic, Voigtlander and Four Thirds Lenses to Micro Four Thirds Cameras and Four Thirds Cameras May 15, 2011

See also

• Digital single-lens reflex camera
• Lenses for SLR and DSLR cameras
• Lens mount
• Micro Four Thirds System