Four Thirds system

"4/3" redirects here. For 4:3 image aspect ratio, see Aspect ratio (image) § 4:3 standard.

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Four Thirds logo

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

The system provides a standard that, 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. This is claimed to be an open standard; it is however only accessible to companies and under a non-disclosure agreement.

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 size of the sensor is slightly smaller than for most DSLRs (see drawing below) and this implies that lenses, especially telephoto lenses, can be smaller. For example, a Four Thirds lens with a 300 mm focal length would cover about the same angle of view as a 600 mm focal length lens for the 35 mm film standard, and is correspondingly more compact. That is, the Four Thirds System is said to have a crop factor (focal length multiplier) of about 2.

Sensor size and aspect ratio

Drawing showing the relative sizes of sensors used in most current digital cameras, relative to a 35mm film frame.

The name of the system comes from the size type of the image sensor used in the cameras. The image sensor is commonly referred to as a 4/3" type or 4/3 type sensor. The common inch-based sizing system is derived from vacuum image-sensing video camera tubes, which are now obsolete. The imaging area of a Four Thirds sensor is equal to that of a video camera tube of 4/3" diameter.

The size of the sensor is 18×13.5 mm (22.5 mm diagonal), with an imaging area of 17.3×13.0 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 compact digital cameras (see image sensor format).

The Four Thirds system uses a 4:3 image aspect ratio, in common with compact digital cameras. 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 8×10, as well as medium format 6×4.5 and 6×7 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 thus 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, but the difference is not great.
• 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

• Since the practical lens aperture for a given angle of view is smaller, the minimal depth of field will be larger, providing less subject isolation.
• The aspect ratio of pictures taken with a Four Thirds camera is 4:3, while all other DSLR cameras and full frame 35 mm film cameras take pictures with an aspect ratio of 3:2. Nearly all compact digital cameras take pictures with a 4:3 aspect ratio.

For traditional print and frame sizes that have an aspect ratio of 3:2 (e.g., 6×4"), photographs will have to be cropped or printed with borders to fit these sizes. The same applies if the picture is to be used for a wide-screen application.

Other traditional print sizes (5×7", 8×10", 11×14") are closer to a 4:3 aspect ratio than they are the 3:2 aspect ratio, meaning the photographer does not need to crop as much as would a user of a 3:2 format. The same applies for pictures to be used on standard PC screens and non-HDTV television screens.

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):

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

This should not be interpreted as a commitment to end user products by each company. Up to now, only Leica, Olympus, and Panasonic have bodies. Olympus, Leica/Panasonic, and Sigma make dedicated Four Thirds lenses (the Sigma lenses are adaptations of their "DC" lenses for APS-C format DSLRs). Kodak, for example, once sold sensors to Olympus for use in their Four Thirds bodies; the newer Olympus Four Thirds cameras use Panasonic sensors.

Four Thirds system cameras

The majority of Four Thirds system cameras (and Four Thirds lenses) are made by Olympus. As Olympus does not incorporate image stabilization technology into its lenses, many Four Thirds cameras utilize "sensor-shift" in-body image stabilization. All Four Thirds cameras also incorporate an automatic sensor cleaning device, in which a thin glass filter in front of the sensor vibrates at 30 kHz causing dust to fall off and adhere to a piece of sticky material below. Anecdotal reports and reviews report that this system is quite effective; few users report any issues with sensor contamination.

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 2006)
• 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)
• Olympus E-30 (November 2008)
• Olympus E-620 (March 2009)
• Olympus E-450 (March 2009)

Four Thirds system lenses

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).

The Four Thirds lens mount is specified to be a bayonet type with a flange focal distance of 38.67 mm.

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:

	Wideangle	Standard	Telephoto	Supertelephoto	Special-purpose
Standard	9–18 1:4–5.6	14–42 1:3.5–5.6	40–150 1:4–5.6	70–300 1:4–5.6 macro	35 1:3.5 macro 18-180 1:3.5-6.3 superzoom
Pro	11–22 1:2.8–3.5	12–60 1:2.8–4 14–54 1:2.8–3.5 25 1:2.8 "pancake"		50–200 1:2.8–3.5	50 1:2 macro 8 1:3.5 fisheye
Top Pro	7–14 1:4	14–35 1:2	35–100 1:2 150 1:2	90–250 1:2.8 300 1:2.8

• Olympus also makes 1.4× and 2× 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

Main article: Micro Four Thirds system

Concept Micro Four Thirds camera by Olympus

In August 2008, Olympus and Panasonic introduced a new format: Micro Four Thirds.

The new system uses the same sensor, but removes the mirror from the camera design. The goal of the new system is to allow for even smaller cameras, competing directly with higher-end point-and-shoot compact digital cameras and lower-end DSLRs. The smaller flange focal distance allows for more compact lenses.

Four Thirds lenses can be used on Micro Four Thirds bodies with an adapter, although not all models can auto-focus (see main article).

See also

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

External links

• Official Four Thirds System site
• Four Thirds US patent 6,910,814; PDF version (1.7 MiB)
• Andrzej Wrotniak's pages about the Four Thirds System - includes a complete lens list
• Spanish site about the Four Thirds System
• Four Thirds User - independent site and user-community dedicated to the Four Thirds system, including Micro Four Thirds