The Four Thirds standard is 100% designed for digital SLR system. By pursuing the optimum relationship between image sensor size and lens mount size, the Four Thirds system successfully combines high image quality and compact size.

Angle of view
Lens resolution & cornershading
Perfect measures
General Advantages
Angle of view

Whoever has used lenses designed for 35mm film with digital camera bodies incorporating sensors smaller than film knows the potential problem: a loss of wide-angle capability. Four Thirds solves this phenomenon. As the lens is always fully optimised to suit the sensor, you don’t have to contend with expensive wide-angle lenses that do not attain their intended specification – the angle of view you expect is the angle of view you get. The standard focal lens of the Four Thirds standard is around 25mm. So to compare for the 35mm format with a standard focal length of around 50mm, you have to use the factor 2 to have the angle of view compared (50mm = 100mm related to 35mm equivalent focal length).  


Lens resolution & cornershading

Because the medium of film is very tolerant, there was no need to equip lenses with very high resolving power in the past. But with digital sensors, this has changed.  
Conventional lens systems simply do not deliver the lens resolution necessary for electronic sensors. They often have a resolution of only around 10 microns. However, because modern multi-megapixel cameras often have a pixel pitch of just 5 or 6 microns or less, overlapping and quality losses are the result. Therefore, the full theoretical performance of the sensor cannot be used. The problem is exacerbated with a greater number of pixels and a smaller sensor size.  
All lenses designed for the Four Thirds standard ensure the image sensor is not wanting. The ultra-fine resolution attained through special manufacturing processes guarantees full sensor performance.  

For edge to edge clarity

All Four Thirds-optimised lenses feature so-called near telecentric optics.

Due to the nature of electronic sensor construction, image data is lost and there is a susceptibility to interference if light hits the pixels at a high angle of incidence. It causes a situation called cornershading, a loss in quality at the edge of the frame. This is especially associated with cameras using full-format sensors, where lenses must spread the light widely to reach each corner.
However, the near telecentric construction of lenses optimised for the Four Thirds standard ensures light hits the sensor at right-angles. This guarantees edge-to-edge colour, clarity and brightness.

Lens designed for 35mm film cameras. Film is tolerant with respect to light hitting it at an angle. Even at high resolutions film can collect light falling at a high angle of incidence without a critical loss of brightness.

Lens designed for 35mm film cameras. Following effects occur when light hitting the sensor at high angles.
1) Reflection onto neighbouring pixels
2) Crosstalk between neighbouring pixels
3) Loss of brightness since some of the light cannot be captured by the sensor.
These effects reduce the signal to a lower signal to noise ratio and poor colour
Four Thirds format:
approx. 18 x 13.5mm (diagonal: 22.5mm)
APS format: approx. 24 x16mm (diagonal: 28.8mm)
35mm format: 36 x24mm (diagonal: 43.3mm)

Near telecentric lens as in ZUIKO DIGITAL lenses. The near telecentric construction of lenses optimised for the Four Thirds Standard ensures light hits the sensor at almost perpendicular angles.
This guarantees edge to edge colour, clarity, and higher brightness.

The standard for the future

Four Thirds is a technological standard dedicated to the needs of digital SLR camera systems. It has been designed to maximise the performance of both image sensors and lenses. To achieve this, it sets definitive standards on three levels:

• mechanical 
• optical and 
• communication


Mechanical standards
The type, size and shape of the bayonet mount and mount bore are standardised to permit lens interchangeability.
Also the distance between the focal plane and the lens flange surface (flange back) and the position of the image sensor in relation to the rear of the lens have been defined.

Optical standards
The Four Thirds standard fixes the size of the image circle, i.e. the diameter of the circle projected by the lens onto the focal plane. The lens mount is roughly twice as big as the image circle, a feature necessary to allow the light to fall at almost perpendicular angles onto the entire image sensor via the near telecentric lens elements, thus virtually eliminating cornershading.
A communication standard
The lens and body are no longer separate entities. Now, through the standardised communications protocol established by Four Thirds, the two can communicate and exchange information.
Perfect measures

Four Thirds is a reference to the size of the image sensor. The image sensor for Four Thirds cameras is what is commonly referred to as a 4/3 type or 4/3 type sensor. These describe the type of sensor not the actual size of the light sensitive area, which is normally much smaller
The sensor measures approximately 22.3mm diagonally, not four-thirds of an inch, which would be about 33.87mm. Traditionally, the nominal size of image-sensing devices has been based on a method of calculation that was introduced when vacuum image-sensing tubes were first invented.
At the time, the outer diameter of these early 'vidicon' tubes was used to indicate their size. Unfortunately, this convention has remained despite the many advances in imaging technology that have since been made, and so the designation, "a four-thirds-inch sensor," can be a source of confusion. For this reason, many people now prefer to use the word "type" instead of "inch" when discussing the size of digital image sensors.
The type, size and shape of the bayonet mount and mount bore are standardised to permit lens interchangeability.
Also the distance between the focal plane and the lens flange surface (flange back) and the position of the image sensor in relation to the rear of the lens have been defined.

This size was chosen by the Four Thirds design team because it optimally met the different requirements: the need for high image quality and avoiding noise, for accommodating different pixel sizes in the future and allowing the use of a near telecentric lens construction.


An open standard

The Four Thirds standard has deliberately been designed as an open standard. That means the more body, lens and accessories manufacturers adopt the standard, the greater will be the degree of product compatibility. As a result, the range of equipment and options available to photographers will be all the greater.

General Advantages

All advantages of the Four Thirds System at one sight:
  • No decrease in image resolution due to insufficient lenses – full use of the sensor’s performance
  • No loss of wide angle capability
  • Significantly reduced occurrences of cornershading
  • Smaller, lighter lenses and bodies
  • Brighter lenses by approximately two f-stops with same depth of field as 35mm lenses (plus solutions to provide shallow depth of field if required)
  • Designed to remain compatible with future sensor developments
  • Cross-manufacturer interchangeability
  • Perfect harmonisation of lens and body due to communication standard
  • Secure investment due to future-oriented design
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