Pekka Buttler, March 2023
Note! This article is about the 2008–today mirrorless camera lens mount. The related SLR mount (the Four Thirds -mount is discussed separately)
Micro Four Thirds – basics and history
“Micro Four Thirds” seems to be too long a name for a lens mount, because not only is it regularly abbreviated, it is abbreviated with gusto (read:haphazardly). Various acronyms used are: MFT, µFT, m4/3 even m43. JAPB will use only MFT.
At the time of its launch, the Micro Four Thirds lens mount was the first modern mirrorless mount. It is also clearly evident that the MFT mount was an evolution of the existing Four Thirds mount: keeping the same sensor size, while offering a significantly shorter flange focal distance in a marginally smaller (physical) but structurally similar mount with two added electronic contacts.
Whether one can say that the MFT mount inspired all the subsequent mirrorless interchangeable lens camera (MILC) mounts, or whether all MILC mounts simply gravitated towards a technologically ideal solution will have to be left to historians. The fact however is, that all current mirrorless mounts are very similar in fundamental design (all are male bayonet mounts, none have any physical lens-body communication, all have an array of electronic contacts and while their radial placement may differ, they are otherwise amazingly similar.)
In any case, at the time of its launch, the MFT launch offered – by a considerable margin – the platform for adapting the widest range of legacy lenses. Hence, one can say that the launch of the MFT system made an early contribution to re-endgendering interest in legacy lenses.
Micro Four Thirds mount specifications
Mount type: Bayonet mount (lens release on camera)
Flange focal distance: 19,25 mm
Sensor size: 17,3 x 13 mm
• electronic only (aperture, autofocus, focusing distance)
• 11 electronic contacts (spring-loaded at the camera’s end)
Adapting Micro Four Thirds lenses
Micro Four Thirds lenses are for the most part thoroughly modern lenses, and depend extensively on electronic camera-lens communication to allow aperture control. That said, there are also some enthusiast manual focus, manual aperture MFT lenses.
Hence the usability of Micro Four Thirds lenses (except on a MFT body) depends on two–three criteria:
• First, a suitable smart adapter must exist if your intention is to adapt an electronic MFT lens. I am not aware of such an adapter currently existing.
• Second, the target camera mount must have a sufficiently shorter flange focal distance than the MFT mount. That list is short (but above zero and growing).
• Third, the target camera must be able to deal with the smaller image circle produced by the MFT lens. Many MFT lenses will be able to produce an image circle that covers an APS-C sensor, but covering Full-frame is out of the question.
These factors together mean that MFT lenses are not prime candidates for adapting onto other systems, and while some MFT lenses will be possible to adapt to APS-C or smaller sensor size mirrorless systems using a dumb adapter (such as Sony E, Fujifilm X, Canon EF-M, Nikon 1 or Nikon Z (DX)), that will not apply to all lenses. In essence, whether an MFT lens can be adapted to another system depends as much on the lens as what that other system is.
Adapting to Micro Four Thirds cameras
When the Micro Four Thirds system was launched in 2008, it was by far the digital camera mount with the shortest flange focal distance. Hence, it opened up a range of legacy lenses for adapting that before the advent of the MFT mount could not be used on digital cameras (such as Pentax 110, Robot, Contax G, Olympus PEN F, Contax/Kiev, Argus, Konica AR …) Furthermore, MFT bodies were able to use Four Thirds lenses using a dedicated smart adapter.
At the same time, the Micro Four Thirds mount grappled with the same issue as the Four Thirds mount before it – namely that the significantly smaller sensor also implied a significant crop factor to the use of adapted (full frame) lenses.
However, and in contrast to the Four Thirds mount before it, the MFT mount had such a short flange focal distance, that (with many legacy lenses) there was enough space to implement specialised adapters, such as tilt-shift adapters, helicoid adapters and speedboosters. Considering the crop factor of MFT cameras, the addition of speedboosters is especially welcome as speedboosters allow the effective crop factor to be somewhat reduced.
In sum, unless the crop factor is an unsurmountable issue, MFT bodies provide a tempting approach to adapting legacy lenses.