FourThirds is the answer to the uncompromising demands of professional photography. It represents a break from the past and frees digital SLR photography from the confines of conventional camera technology.
When you use lenses designed for 35mm film with digital camera backs that incorporate sensors smaller than film there is the potential problem of a loss of wide-angle capability.
FourThirds 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 FourThirds 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).
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 FourThirds standard ensure the image sensor is not left wanting. The ultra-fine resolution attained through special manufacturing processes guarantees full sensor performance.
For edge to edge clarity
All FourThirds-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 corner-shading, a loss in quality at the edge of the frame. This is associated particularly 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 FourThirds 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.
The following effects occur when light hits 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. When combined, these problems lead to increased noise and poor results.
The near telecentric construction of lenses optimised for the FourThirds Standard ensures light hits the sensor at almost perpendicular angles. This guarantees edge to edge colour, clarity and higher brightness.
FourThirds 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 inter-changeability.
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 FourThirds 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 corner-shading.
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.
FourThirds is a reference to the size of the image sensor. The image sensor for FourThirds 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 CCD 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 FourThirds 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 FourThirds 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 greater.
