It hasn't been long since the first digital SLR camera was introduced to the market. However, the restrictions imposed by having to look into a viewfinder and the limitations on size/weight reduction have led to the need for a new breed of SLR camera - one that is smaller, leaner, and easier to use. This has led to support for Live View shooting and rising demand for high-performance compact machines.
In consideration of these new trends in the market, the Micro Four Thirds Specification was developed and standardized to provide a basis for an interchangeable-lens type camera system that would expand the potential of digital photography and make it more accessible to more users.

The Micro Four Thirds Specification was developed with the following objectives.

(1) To extend the Four Thirds Specification by reducing the size and weight further, while maintaining the high picture quality of the 4/3-type image sensor device.

(2) To develop a standard with a basic structure that can deal with changes over time such as acceleration of the need for further size reduction or advancement of Live View photography style.
 - Live View dedicated specification with a structure eliminating the mirror box.
 - About half the flange back length of the Four Thirds System.
 - Reduced lens mount diameter without compromising strength.
 - Addition of contacts to improve the speed and efficiency of lens-body data exchange and control.

(3) To enable use of existing Four Thirds System lenses.
 - Compatibility is provided by a dedicated mount adapter.
* Since the Four Thirds camera body has a longer flange back length than the Micro Four Thirds camera body, Micro Four Thirds lenses cannot be mounted on Four Thirds camera bodies.

(4) To facilitate the addition of movie capability in the future by providing compatibility with image aspect ratios from 4:3 to 16:9 within the diagonal length of the effective pixel area of the Four Thirds Specification.

The Four Thirds System refers to an interchangeable-lens type photography system based on an imaging sensor with a diagonal length of 21.63 mm, which is half the diagonal length of the 135 film format (36 mm x 24 mm).
The Micro Four Thirds System refers to an interchangeable-lens photography system with a shorter flange back length than the Four Thirds System. It uses an image sensor matching the Micro Four Thirds System (Micro Four Thirds System Sensor), and is an extension of the Four Thirds System.

The Micro Four Thirds System lens mount is the generic name given to the section where a lens or other accessory is connected to the camera body, and includes the physical, electrical and optical interfaces as well as various communications specifications.

●Differences between Four Thirds System mount and Micro Four Thirds System mount: 1
Mount diameter reduction
As a result of research aimed at facilitating the design of compact, lightweight lenses while maintaining the current strength, the outer diameter of the lens mount has been reduced by approx. 6 mm.

●Differences between Four Thirds System mount and Micro Four Thirds System mount: 2
Addition of signal contacts
To facilitate the movie capability that may be required in the future, two signal contacts have been added to enhance and extend the basis for data exchange.
This has resulted in an improvement in the processing speeds of the entire camera system including accessories and the provision of a basis for manifesting stable control capabilities.
In the review of the signal contacts, accessory sequences, as well as the signal exchange between the lens and body, were also reviewed in order to improve the data exchange speed by creating a more rational flow.

About the reduced flange back length
With the SLR camera system, the setup of the flange back (distance between the lens mount plane and the film/image sensor surface) is very important as it aligns the focal point of different interchangeable lenses and ensure that focusing is possible. Conventional SLR cameras that use a pentaprism type viewfinder require a mirror box, which means that the flange back is longer than that of compact cameras that do not use a mirror box.
Because of the long flange back, it is very difficult to reduce the size and thickness of conventional SLR cameras.
In addition, current digital SLR cameras incorporate many components in front of the image sensor, such as the image stabilizer, dust reduction mechanism and low-pass filter. This all adds to the difficulty of making these cameras slim enough to meet market demand.
However, growing support for Live View shooting in the market presented another option. We could eliminate the conventional viewfinder and therefore the mirror box altogether. On this basis, the Micro Four Thirds System was conceived. It specifies the optimum flange back length required to reduce camera size and thickness, assuming the omission of the mirror box. The flange back length has been reduced to about 1/2 that of the Four Thirds System. This is considered short enough to allow radical reductions in size and thickness, without causing problems with the lens drive and while still reserving space for accommodating the devices indispensable for digital SLRs such as the low-pass filter and the dust reduction mechanism, as well as devices likely to be adopted in the future.
This, together with the reduction of the lens mount diameter, has made it possible to reduce the thickness and size of SLR cameras without compromising the high picture quality.

Aspect ratio policy of Micro Four Thirds System
Covering the 16:9 aspect ratio to facilitate movie capability in the future
The aspect ratio (diagonal length of the effective pixel area) refers to the ratio between the horizontal size and vertical size of the rectangle inside the image circle for use in recording and processing signals. The Micro Four Thirds System is designed to be compatible with the aspect ratios specified for the Four Thirds System, including 4:3, 3:2 and 16:9.

Compatibility between Micro Four Thirds System lenses and Four Thirds System lenses
Availability of existing Four Thirds System lenses with an adapter
Although the best way to benefit from the advantages of the Micro Four Thirds System is to use the dedicated lenses, the existing Four Thirds System lenses can also be mounted on a Micro Four Thirds System body by using an adapter.

* When a Four Thirds lens is mounted on a Micro Four Thirds body using an adapter, all of the functions of the Micro Four Thirds System may not always be available.