Saturday, July 11, 2009

Four-Thirds System – What it means?

Authors Note: This blog is compilation of information from various sources. I have tried to make it simple and interesting. Since I have tried to make it simple, its very difficult for me to demark exactly which information comes from which source. The reader will see the information blended from all these sources.

What is four-thirds system?

The Four Thirds system is a standard created by Olympus and Kodak for digital single-lens reflex camera (DSLR) design and development. The system provides a standard that, with digital cameras and lenses available from multiple manufacturers, allows for the interchange of lenses and bodies from different manufacturers.

Unlike older SLR systems, Four Thirds has been designed from the ground up to be entirely digital. Many lenses are extensively computerized, to the point that Olympus offers firmware updates for many of them. Lens design has been tailored to the requirements of digital sensors, most notably through telecentric designs. The size of the sensor is slightly smaller than for most DSLRs (see drawing below) and this implies that lenses, especially telephoto lenses, can be smaller. For example, a Four Thirds lens with a 300 mm focal length would cover about the same angle of view as a 600 mm focal length lens for the 35 mm film standard, and is correspondingly more compact. That is, the Four Thirds System is said to have a crop factor (focal length multiplier) of about 2.

[Outer frame is Medium format (Kodak KAF 3900 sensor)]


What does this mean?

The name of the system comes from the size type of the image sensor used in the cameras. The image sensor is commonly referred to as a 4/3" type or 4/3 type sensor. Its area is 30–40% less than the nearly APS-C sensors used in most other DSLRs, but around 9 times larger than the 1/2.5" sensors typically used in compact digital cameras (see image sensor format).

The Four Thirds system uses a 4:3 image aspect ratio, in common with compact digital cameras. This differs from other DSLRs which usually adhere to the 3:2 aspect ratio of the traditional 35 mm format.

A major reason to choose 4:3 sensor proportions was similarity to the aspect ratio on standard definition television. Computer monitors also commonly use a 4:3 aspect ratio, as found in the VGA, SVGA, XGA, SXGA+, UXGA and QXGA standards.

John Knaur, a Senior Product Manager at Olympus, states that "The Four-Thirds refers to both the size of the imager and the aspect ratio of the sensor".He goes on to state the similarity between 4:3 and the standard printing size of 8×10, as well as medium format 6×4.5 and 6×7 cameras.

What are the objectives?

Olympus developed the Four Thirds Specification with the following objectives.

  • To propose an interface specification that will allow more camera bodies and lenses to be interchangeable, in order to improve convenience of users and contribute to the growth of industry.
  • To pursue higher image quality than 135 film cameras and mobility through the effect of size reduction.
  • To consider free system expansion.

How does it work?

The image sensor in a digital camera can be compared to a “deep well.” You cannot see the bottom of the well unless you lean over it. In the same way, light inclined at an angle cannot reach the image sensor (i.e. the bottom of the well). Many of the current interchangeable-lens D-SLR cameras using traditional 35 mm film camera lenses are very susceptible to loss of sharpness, chromatic aberration, and shading of peripheral areas. Wide-angle type lenses are especially problematic since oblique light inclined at a large angle tends to enter the peripheral areas.



With the Four Thirds system, the diameter of the lens mount exceeds the sensor size and the digital-dedicated lens design allows all the light (even on the periphery) to travel perpendicularly to the surface of the image sensor. The result is a sharp, clear image reproduction throughout the image plane.
Linear propagation of light and the high imaging performance made possible by the digital-dedicated design are the biggest features of the Four Thirds lenses.

So, What are the advantages?

  • The smaller sensor size makes possible smaller and lighter camera bodies and lenses. In particular, the potential exists for very fast lenses and very high quality lenses at lower costs. Currently this is evident to some extent in the Olympus E-4x0 and E-620 bodies, in the kit lenses sold with the E-4x0 and E-5x0 bodies, and in longer telephoto lenses.
  • Greater depth of field at any given aperture and focal length due to the smaller format. This is an advantage in many applications (macro-photography in particular), but a disadvantage in others (portraiture) where a shallow depth of field is desired.
  • Telecentric optical path means that light hitting the sensor is traveling perpendicular to the sensor (as seen in the figure above), resulting in brighter corners, and most importantly improved off-center resolution, particularly on wide angle lenses.
  • Advantage of 2x crop while shooting with telephoto. A 300mm 4/3rd lens is equivalent of 600mm for 35mm format.
  • Because the flange focal distance is significantly shorter than most competing mounts (such as Canon FD, Canon EF, Nikon F and Pentax K), lenses for many other SLR types, including the old Olympus OM System, can be fitted to Four Thirds cameras with simple mechanical adapter rings. (Such mechanical adapter rings typically require manual setting of focus and aperture). In many cases this produces excellent results, especially with longer focal-length lenses and lenses at smaller apertures. A series of tests by John Foster (Using OM legacy lenses on E1 body) provides a demonstration.

And the disadvantages?

The disadvantages are not really disadvantages with the development of advanced technology, as you can see

  • Smaller sensors collect less light in total than larger ones, and thus have a weaker signal-to-noise ratio. Images made with a Four Thirds sensor will show more noise at the same sensitivity than those made with larger formats, although the noise performance of Four Thirds is only slightly worse than APS-C.  However, with advanced image processor in the E-systems, Olympus has almost eliminated this disadvantage.
  • A telecentric optical path means more aggressive retrofocus design for wide and normal lenses, which makes them bigger, and makes wide apertures harder to achieve. With latest high-tech lenses (especially the 9-18mm and 12-60mm lenses, not to talk of their super high-grade lenses) Olympus is now on par with the competitors.

Other Factors

  • Since the practical lens aperture for a given angle of view is smaller, the minimal depth of field will be larger, providing less subject isolation.
  • The aspect ratio of pictures taken with a Four Thirds camera is 4:3, while all other DSLR cameras and full frame 35 mm film cameras take pictures with an aspect ratio of 3:2. Nearly all compact digital cameras take pictures with a 4:3 aspect ratio.
  • For traditional print and frame sizes that have an aspect ratio of 3:2 (e.g., 6×4"), photographs will have to be cropped or printed with borders to fit these sizes. The same applies if the picture is to be used for a wide-screen application.
  • Other traditional print sizes (5×7", 8×10", 11×14") are closer to a 4:3 aspect ratio than they are the 3:2 aspect ratio, meaning the photographer does not need to crop as much as would a user of a 3:2 format. The same applies for pictures to be used on standard PC screens and non-HDTV television screens.

For more information on how Four-third system was born – You can read these interesting stories