Micro Four Thirds is a photographic system, similar in concept to Olympus' OM system for film photography. The name 'Four Thirds' derives from the size and format of the image sensor used in the camera bodies. The system was developed by Olympus in partnership with Panasonic.
It is based on the Four Thirds system for digital SLRs, but lacks a mirror box, being thus adequate for compact cameras. It has half the focal flange distance and 6mm smaller bayonet throat size.
Four-Thirds is not an open standard; Olympus controls access to the design specifications and no unauthorised third-party companies are able to use the Four-Thirds logo or create supported peripherals for the system.
Unlike 35mm film (and therefore most APS-C sized sensors used by other big camera brands such as Nikon, Canon or Pentax) the Four-Thirds system uses a ratio of 4:3 for its image sensor — 35 mm film has a ratio of 3:2 (whereby the long edge of the rectangular sensor is either 4/3rds or 3/2 the length of the shorter side). This ratio was inherited from analogue TV standards.
The actual size of the 4/3 sensor — specified by Olympus, and a design constant in the 4/3rds system white paper — is 18×13.5 mm, with an imaging area of 17.3×13 mm. This gives the image a diagonal length of 21.6 mm; the smallest of the common APS-C sensors used has a diagonal of 24.88 mm with dimensions of 20.7×13.8 mm.
The diagonal length of the 4/3rds sensor is roughly half that of a 35 mm film negative. The Four-Thirds system is often criticised for the small size of its sensor; referral to the diagram to the right will show that the sensor size is only very slightly smaller than the currently most popular entry-level DSLR, the Canon EOS 400D. Four-Thirds lenses are specific for their own bodies, and planned specifically for the digital technology, but cameras can use all lenses made for longer register distances. Systems composed of pseudo–APS‐C and 24×36 mm camera and lenses, on the other hand, can use only their specific lenses, some of which are not adequate for digital technology, while its 24×36 mm lenses will also work with same-system pseudo–APS‐C bodies, but pseudo–APS‐C lenses will not work for 24×36 mm.
The ratio of the image size, combined with the smaller sensor means that Four-Thirds based DSLRs have a 'crop factor' of exactly 2 — that is to say, a 50mm film lens used on a Four-Thirds body gives the equivalent field of view to a 100mm lens on a 35mm film camera body. The term 'crop factor' is somewhat misleading as the sensor does not crop the image at all — it simply utilises a smaller section of the lens. In essence only the portion of light entering the lens that actually hits the sensor is altered, meaning the field of view is smaller, and thereby effectively doubling the focal length of the lens.
The 'crop factor' does affect the apparent depth of field of the taken image: with the magnification of 2, the DOF is effectively doubled (again, it is important to note that this perceived effect is due to the FOV 'crop'; the same depth of field is present, but you are viewing it at twice the magnification).
It is important to note that neither the light gathering ability of the lens, nor any of its traits are actually affected: a 50mm f2 lens on a 4/3rds body has the same f-stop and depth of field as a 50mm lens on a 35mm camera, but the 4/3rds sensor only records the equivalent field of view of a 100mm lens. As a general rule of thumb the depth of field of a lens is halved as its focal length is doubled (if the aperture remains constant). The end result is therefore that a 4/3rds camera has twice the depth of field as a 35mm camera with a lens covering the same field of view.
Returning to the above example, a 50mm f/2 OM lens used on a 4/3rds digital body is effectively equal to a 35mm camera with a 100mm lens set at f/4.
As a further example of the above, let us compare a 35mm camera and a 4/3rds camera in a given situation:
- 35mm: 100mm lens, f/4, ISO 800, 1/200sec
- 4/3rds: 50mm lens, f/2, ISO 200 1/200sec
Using the same shutter speed, both these cameras would have the same exposure, have the same depth of field, and the same field of view. The smaller sensor size of the 4/3rds allows a lower ISO to be used at the same shutter speed; the doubling of the depth of field allows a two-stop advantage in sensitivity for the same end result.
This allows a photographer to either use a lens at a lower F-stop (for faster shutter speeds) or shoot at a lower ISO (for lower noise in the final image) than would be possible in 35mm or APS-C format. Olympus states this field of view/depth of field effect was a primary consideration in the design of the Four-Thirds system.
The principles outlined above in the design of the 4/3rds sensor also follow into the lens designs for the system: with a crop (or magnification) factor of 2, a telephoto lens for the 4/3rds system with the same field of view that of a lens designed for 35mm can be half the size.
For example, a 150mm lens fitted to a 4/3rds body produces the same field of view (and therefore effectively the same magnification) as a 300mm lens on a 35mm film camera. Not only does this mean 4/3rds lenses can be half the size of their 35mm counterparts, it also means it is much easier and cheaper to produce lenses with good focal length to f-stop ratios.
Most 4/3rds lenses are also telecentric, meaning that no matter what angle the light enters the lens, the glass within the lens is designed to ensure the beams of light hit the sensor straight on. This design requirement is due to the nature of digital sensors: they are highly sensitive to the angle at which light hits them, with highly angled photons not being picked up by the sensor at all.
The lens mount for 4/3rds is roughly twice that of the imaging sensor, again allowing improvements in lens design: with the lens focusing the image down to half the area before it hits the sensor, it allows the lenses to be brighter without sacrificing the size or weight advantages gained by the smaller sensor.
Information on 4/3rds mount lenses is listed on the Four-Thirds lenses page.
To date, two companies have commited to products compatible with the Four-Thirds system: Olympus and Panasonic.