Four Thirds

Four Thirds is an interchangeable-lens camera standard introduced in 2003 by Olympus and Kodak. It was one of the first systems designed from the ground up for digital photography rather than adapted from a film camera standard. Four Thirds cameras use a 17.3 x 13mm sensor — the same physical size as Micro Four Thirds — but in a traditional DSLR-style body with a mirror box, an optical viewfinder, and a larger lens mount diameter. The mount flange distance is 38.67mm, compared to 19.25mm on Micro Four Thirds.

The Four Thirds lens mount is physically larger than the M43 mount and cannot accept M43 lenses directly. Four Thirds lenses can be used on M43 cameras via the Olympus MMF-3 or MMF-2 adapter, but autofocus performance is significantly degraded on most lenses. Because Four Thirds lenses were designed for contrast-detect AF with a mirror box, they use slow screw-drive AF mechanisms that are not well suited to the phase-detect systems in modern M43 bodies. Some lenses focus acceptably for still subjects but are generally unsuitable for tracking moving subjects.

Olympus produced a range of Four Thirds lenses under the Zuiko Digital brand, including high-performance options like the 14-35mm f/2, 35-100mm f/2, 150mm f/2, and 300mm f/2.8. These lenses are prized for their optical quality and can be used on M43 bodies for manual focus or slow-moving subjects. The Four Thirds system was discontinued by Olympus in favour of Micro Four Thirds, with the last Four Thirds body produced around 2013.

The 2x crop factor of Four Thirds is identical to Micro Four Thirds. A 150mm f/2 Four Thirds lens gives the same 300mm full-frame equivalent field of view on an M43 body as on a Four Thirds body. This makes the adapted use of high-quality Four Thirds telephoto glass a practical option for photographers who prioritise image quality over autofocus performance.