Four Thirds Lenses

Four Thirds DSLR standard, introduced in 2003 | Four Thirds digital sensor

Four Thirds was a digital-first DSLR standard created around the 4/3-type sensor rather than inherited from a film mount. Olympus led the camera and lens system, with support from Panasonic, Leica-branded lenses, Sigma lenses, and a smaller group of standard participants. Because the sensor size, mount, and lens line were planned together, the system could define its own normal focal lengths and coverage expectations instead of cropping an older 35 mm standard.

The mount kept a reflex mirror box but used a smaller image circle than APS-C or full-frame systems. Many lenses emphasized telecentric light delivery, digital correction expectations, and compact telephoto reach, while professional Zuiko Digital zooms pushed hard on speed and weather-sealed performance. The smaller format also made lenses such as fast 2x-equivalent telephoto zooms more practical than they would be in full-frame SLR form.

In the catalog, Four Thirds is distinct from Micro Four Thirds even though the sensor format is shared. Four Thirds lenses are DSLR-era optics with a longer register and mirror clearance, while Micro Four Thirds lenses use a shorter mirrorless geometry. Adaptation between the systems is possible, but the optical packaging assumptions are visibly different.