Overview of Four Thirds Lenses
Developments in advanced lens design have arguably been at the center of Four Thirds. Olympus and Panasonic conceptualized their lenses for the new imaging system from the ground up for digital photography. In particular, they chose to produce lenses that are telecentric, so that the light rays hit the digital sensor orthogonally. As a result, FT-lenses show a very high and even sharpness from corner to corner. Also, FT-lenses tend to reach a high level of contrast and sharpness already at their widest aperture, so that there is little need to stop the lens down in order to improve image quality.
Moreover, the relatively small sensor in FT cameras has made it possible to reduce the size and weight of 43-lenses, so that many optics are more compact than corresponding lenses for APS-C or full frame systems that provide the same angle of view. Four Thirds lenses have a format factor of 2, meaning that an FT-lens with a focal length of 25mm provides the same field of view as a 50mm lens on a 35mm-full frame camera. Moreover, the Four Thirds lens delivers a depth of field that is 2 aperture stops wider than a 35mm-full frame lens with the same angle of view, so that it is easier to get the main subject in sharp focus. On the other hand, using FT-lenses makes it more difficult to achieve a very narrow depth of field that is sometimes desired in portrait photography.
Since the launch of the FT-system in 2003, the lens catalog expanded quickly over the following years. It grew to comprise a total of more than 50 optics with native FT-mounts. In 2008, Micro Four Thirds was created as a mirrorless camera system with an even shorter flange-focal-distance, thus making it possible to realize substantial further reductions in lens size and weight. While mFT quickly superseded FT, all FT-lenses can be operated on mFT cameras with full aperture control when using an FT to mFT adapter. However, the autofocus performance of FT-lenses will fall short of that of native mFT lenses on mFT cameras, unless the camera has on-sensor phase-detect pixels (like the OMD-EM-1) or the FT-lens has an Imager-AF module designed to work with contrast-detect autofocus systems (like the newer Olympus 2.8/25mm, 4-5.6/9-18mm, 3.5-5.6/14-42mm, 2.8-3.5/14-54mm II, 4-5.6/40-150mm, and 4-5.6/70-300mm).
Olympus released a total of 24 FT-lenses, Panasonic 4, Sigma 14, and Samyang 15. Three of the 57 optics are revised versions of earlier lenses: The Olympus 50-200mm SWD received the silent and quick Supersonic Wave Drive (SWD) autofocus motor in 2007, the Olympus 14-54mm II was upgraded with the Imager-AF module in 2008, and the Samyang 85mm was redesigned with internal focus mechanisms. Production of the original lens-versions stopped when the new designs arrived.
Olympus grouped its Four Thirds lenses into three categories: Super-High Grade (SHG), High Grade (HG), and Standard Grade (SG). SHG lenses are optically excellent primes and constant maximum aperture zooms, including the world's first interchangeable f/2 zooms (the 14-35mm and the 35-100mm). They are of exceptional build quality and feature environmental seals against dust and moisture. The HG lenses are also weather sealed and provide very high optical performance, with the 12-60mm and the 50-200mm being particularly popular among enthusiast photographers for their versatility and imaging quality in a compact package. The SG lenses lenses are affordable, highly portable optics that nevertheless can offer very high optical performance. For example, the 9-18mm or the 35mm Macro are very well regarded as quality lenses at budget prices. Olympus officially discontinued its FT lens line in March 2017.
None of the Sigma or Samyang lenses were custom-made for FT, but represent modified versions of their APS-C or full frame lens offerings. They do not provide a telecentric design, but as they are conceived to cover a larger imaging circle, only the central, "sweet spot" of the lens is used on FT-cameras, so that aberrations towards the borders and edges of the projection frame are not visible. The Sigma lenses support autofocus, while the Samyang offerings (also sold under the Rokinon brand name) require manual focusing.
Due to the relatively short flange-focal-distance of 38.67mm, lenses from many other camera systems can be mounted and used on FT-cameras via adapters. These adapters (see here on eBay) are purely mechanical devices that do not transmit any exposure or focusing information between the lens and the camera, so that aperture and focus have to be set manually. However, some adapters have a "dandelion" chip installed, which reveals lens-specific EXIF data to the camera, makes it possible to use AF confirmation, and indicates the correct focal length, so that the in-body Image Stabilization can work properly. Adapters corresponding to lenses for Olympus OM, Leica R, Nikon F, Minolta MD, M42, Pentax K, and Yashica/Contax exist. There are also adapters for Canon FD lenses, but these adapter-lens combinations do not achieve infinity focus (as the flange-focal-distance of Canon FD is only 3.3mm longer than FT, and a sufficiently robust adapter needs to be thicker), so that they are mainly of interest for macro photography. Also, the bayonet mount and flange distance of Konica AR lenses is very similar to FT and some photo enthusiasts have converted Konica Hexanon lenses for use on FT-cameras.
Below is a grouped listing of all lenses with a native Four Thirds mount and their core specifications. The manual Peleng 3.5/8mm fisheye lens (here on eBay), is included, even though it is mounted onto FT-cameras via an M42-to-FT mount adapter and thus strictly speaking not a native 43-lens. However, it has been quite popular with Four Thirds users trying to explore the fisheye perspective at a reasonable price and, thus, deserves honorable mention in this compendium.
Olympus as the lead company within the Four Thirds consortium used to publish a lens roadmap to give FT users some visibility on upcoming releases. All the envisioned lenses were launched, except one. A 100mm high-grade macro lens featured over several years on the roadmap. Yet, that lens never materialized.
As mentioned earlier, all FT lenses can be mounted on and used with m43 cameras. Corresponding adapters are available from Olympus, Panasonic, or third party manufacturers. You can also find a compendium of all native m43 lenses on this site. Moreover, if you are interested in how a particular camera-lens combination looks like, check out the simulation tool at four-thirds.org. Finally, if the terminology and jargon around Four Thirds seems overwhelming, have a look at the Four Thirds glossary for explanations of key terms.