Four Thirds system

The Four Thirds Standard (or Four Thirds ) is a standard developed by the company Olympus and Kodak, specifically matched to digital SLR standard for lens mounts. It was from the start no regard for conventional lenses, cameras or sensor sizes. This allowed both compatibility problems with old components and compromises in the design of new components are bypassed. The first such camera, the Olympus E-1, was introduced in autumn 2003.

The standard defines, among other things, the sensor dimensions, the lens mount together with the communication protocol, the imaging circle and the flange focal length ( 38.85 mm ), but also optical requirements such as the maximum angle between incident on the sensor light beams.

Origin of the name

The name derives from the standard external dimensions of the sensor (not the effective screen size ), defined according to the traditional methods used in nominal size of image sensors based on the size of the corresponding vacuum imaging tube, the diameter indicated the corresponding tube in inches.

When a Four Thirds image sensor, the image size is therefore the size of the image that four-thirds inch can on the photosensitive surface of an imaging tube are shown with the outer diameter. Typically, in such tubes the diameter of the usable area was about two-thirds of the outer diameter, which explains the sensor diagonal of just 22 millimeters. With an aspect ratio of 4:3 naming Four Thirds (English " Four Thirds " ) has nothing to do.

Sensor size

The screen size of Four Thirds sensors is standardized and is similar to those of the Micro Four Thirds standards. It amounts to 21.633 mm, exactly half the diagonal of the small aspect ratio ( 43.267 mm), corresponding to a crop factor of 2.0. Lenses with focal lengths 20 to 25 mm so that for this format standard lenses.

Most cameras have a sensor with an aspect ratio of 4:3 and an optically active area of 17.31 mm × 12.98 mm (area 224.64 mm ²).

The Panasonic DMC -GH1 and DMC -GH2 have an oversized multi -aspect sensor ( multi-format sensor ) with 18.89 mm × 14.48 mm. This allows for the aspect ratios

  • 4:3 ( 4608 × 3456 pixels, 17.31 mm × 12.98 mm),
  • 3:2 ( 4752 × 3168 pixels, 17.85 mm × 11.90 mm) and
  • 16:9 ( 4976 × 2800 pixels, 18.69 mm × 10.52 mm)

The use of the maximum diagonal (Data for the DMC -GH2 ).

Licensee

Due to an open licensing policy, it is up to each manufacturer to bring the standard related accessories for the system on the market. Compatible components from different manufacturers can be combined with each other.

The first product available on the market was in 2003, when in addition to Olympus only two providers Fujifilm and Kodak supported the new standard, the Olympus E-1. In 2004, with Panasonic, Sanyo and Sigma added three other companies. The following additional manufacturer support the standard official: Astro Design, Cosina, Leica Camera, Carl Zeiss, Schneider- Kreuznach, Tamron and Tokina.

Development

Beginning of August 2008 was presented by Panasonic and Olympus as a further development of the FourThirds system standards called the Micro Four Thirds standard, the compact camera systems with interchangeable lenses are available. The new standard uses a lower flange and a narrower bayonet mount, which makes its employment for SLR cameras virtually impossible.

With a mechanical adapter and because of the electronic compatibility, however, the re-use of lenses of the Four Thirds standards is possible, although the Micro Four Thirds standard with eleven electrical contacts comprises two additional, intended for video applications contacts.

Important features

Demands on the lenses

Objective of the Four Thirds standards have basically the bayonet of the system. Furthermore, there are limits for the fanning out of the exiting light beam from the lens. This should be as parallel as digital image sensors (as opposed to chemical film ) have their full sensitivity only vertically incident light (see also telecentric ). The standard required image circle is in proportion to the sensor size is relatively large in order to achieve a uniform illumination of the sensor. These specifications counteract especially at wide angle lenses and at open aperture vignetting of the image.

As with most digital cameras differs from the sensor size of the dimensions of the 35 - mm - mm film, which serves as a reference because of its once very widespread. Due to its smaller size results in respect of framing an apparent doubling of the focal length.

Through the required smaller picture area and high-speed lenses have smaller proportions, which is reflected by a lower weight.

Depth of field

The depth of field of a Four Thirds lens on a Four Thirds sensor is about twice as large as for sensors with small size at the same f-number and the same perspective and the same image.

More specifically corresponds to the depth of field of a Four Thirds lens with f = 150 mm with an F-number f / d = 2.8 exactly the image of a small lens on a small image sensor with f = 300 mm at f / d = 5.6.

To use a small picture format corresponding to the depth of objects or people against a diffuse background with Four Thirds " stand out", you have to fade in with this sensor format two stages. A Four Thirds lens thus forms already at f2.0 the area sharply, the signs of a miniature lens sharp until f4.0. However, since the FT lenses of the upper class are larger only by one stop as equivalent lenses for the small picture format, the Four Thirds format thus does not quite reach the exemption potential of the larger KB sensors at open aperture. The greater depth of the FT- format, however, is always advantageous if several persons or objects to be photographed, which are not on the same focal plane (eg a group photo in multiple rows ) or when the background is to be included in the screen layout. Desirable is the gain in depth in macro photography, as with the proximity to the object also decreases the depth of field.

Noise and dynamic resolution

Since the signal level of an (ideal ) sensor element to the amount of incident light, and thus ( with constant light density) and is proportional to proportional to the area of the sensor, with a smaller sensor, the dynamic resolution is lower. The necessary reinforcement, among other undesirable thermal noise is also amplified, which means that smaller sensors tend to show a poorer signal -to-noise ratio.

Among the SLR Four Thirds sensor is currently the smallest. The competing SLR cameras with APS-C sensors have sensors with about 56 percent ( Canon EF -S) and 78 percent (Nikon DX, Pentax, Sony DT) more space. Sensors in the 35 - mm camera format does have about 4 times the size. Compared with the sensors of most compact cameras, a Four Thirds chip but still an approximately 5x (1/ 1, 7 "sensors ) to 16x ( 1/3 " sensors ) as large area.

If a motif of a certain brightness mapped at a predetermined angle, this is lower by about 20 percent lens focal length is necessary as, for example, a camera with an APS - C sensor of Four Thirds. Comparing appropriate lenses with the same f-number, the actual aperture is thus smaller by 20 percent in the case of Four Thirds, which corresponds to a reduction in the amount of incident light by about a third. This has to be reduced by about 30 percent dynamic resolution experience, coupled with an increase in the noise floor by about 50 percent. It should be noted that were left in this estimate differences in sensor technology between the camera systems except eight ( the actual performance differences between two specific camera models can therefore significantly lower or even sometimes much larger turn out ).

In practice, the reduced dynamic resolution usually plays a minor role, since the values ​​obtained are still outside it, which can play a phrase or a JPEG image. The increased noise disturbs However, when the surroundings are dark no flash to be used, and instead must work with a high ISO setting, so a high gain. It may also be of importance in recording situations, to be where in subjects with very large image dynamics (actually underexposed) parts reworked and brightened.

One can compensate that - as well as the increased depth of field - by lenses with a better two f-stops f-number than the small picture format.

Communication between the camera and accessories

A central feature of the system are intelligent components which communicate via electrical contacts with each other via a standard also defined protocol.

In this way, focus data, Aperture and focal length as those of competing systems will be transmitted electronically between camera and lens. The Four Thirds system may also transfer characteristics of the lens, such as characteristics of the distortion or vignetting at the camera, which allows for digital compensation of aberrations.

Backward Compatibility

Since this is a new development at the Four Thirds, initially exists no complete backward compatibility with other systems. In addition, the previous manufacturer Olympus OM system a purely mechanical bayonet without automatic focusing or aperture control is used, so no consideration had to be taken on compatibility here. OM lenses can be operated via an adapter ring on the Four Thirds mount. The same applies to a number of other non-system lenses.

Camera housing

Previously presented models of the Four Thirds standards in order of their presentation to the public are:

1: Dust and splash protection 2: with Live View 3: in the case of integrated image stabilizer 4: full support for the Optical Image Stabilizer OIS Panasonic 5: Display on the horizontal axis folding 6: display to the vertical axis and the horizontal pivot axis for rotation

A complete overview of all camera housing currently available can be found on the product pages of the Four Thirds consortium.

Lenses

Explanation: SWD / HSM = lens with ultrasonic drive, OIS = Lens with optical Image Stabilizer

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