The purpose of the Four Thirds System Specification (Four Thirds Specification) is to standardize an interchangeable lens system for digital cameras. The Four Thirds Specification provides users reliable and extensive variations of products suitable for digital age. 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 background, summary, and features of the Four Thirds Specification are described below.

2. Background

With the advancement of digital technology in recent years, there are image sensors used on 135 film single-lens-reflex cameras. However, the optical design of the current interchangeable lens for the 135 film camera is not perfect for the image sensor. In light of this situation, the Four Thirds Specification proposes an optimum image sensor size and specifications of interchangeable lens suitable for camera in the digital age.

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3. Objectives

The Four Thirds Specification was developed with the following objectives.
(1) To propose an interface specification that will allow more camera bodies and lenses can be interchangeable, in order to improve convenience of users and contribute to the growth of industry.
(2) To propose an interface specification that will allow more camera bodies and lenses can be interchangeable and contribute to the growth of industry.
(3) To pursue higher image quality than 135 film cameras and mobility through the effect of size reduction.
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4. Features

(1) Size of the 4/3-type Sensor The standard diagonal length of the sensor is 21.63 mm. It is half that of 35-mm film format (36 mm x 24 mm) and suitable format for professional use in digital age. The image circle of the interchangeable lens is specified based on this diagonal length. The focal length is about a half that of a 135 film camera lens assuming the same angle of view.
(2) Any Lens Any Body. (Guarantees the functionality of lens interface, focus, exposure control, etc.)

5. Construction

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6. Four Thirds Sensor Specifications

Specifies the recording pixel area and others.
The standard diagonal length is 21.63 mm (half that of 35-mm film format).

7. Four Thirds Mechanical Interface Specifications

Mount: Bayonet mount (three pawls).
(70 degrees attachment angle with a stopper to prevent rotation)
Lock mechanism: Lock groove on the lens-mount surface.
Movable lock pin on the body engages when mounting is complete.
Electric contact: Fixed contact on the lens, movable contact on the body (9 contact points).
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8-1. Four Thirds Electrical Interface Specification

Lens information is collected through communication when the lens is mounted.
Identification system with vendor ID information is defined.

8-2. Communication Interface

At lease one central processing unit (CPU) is set in each of camera body and lens.
CPU in the lens drives the actuator built in it according to the signal sent from CPU in the body or the lens operation initiated by the user.
The body supplies the power to the lens.

8-3. Communication Specifications

The CPU in the body controls the supply of the power and does periodic communication to detect the lens attached.
CPU in the body takes lens information through communication and controls the lens according to its characteristics.
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