Four Thirds and Micro Four Thirds
2003 (Four Thirds); 2008 (Micro Four Thirds)
An open standard for a 17.3×13 mm sensor, designed for digital from the start. Its Micro version, without a mirror, started the mirrorless camera.
How it began
Most digital SLRs used film-era lens mounts. Olympus and Kodak proposed a standard designed for digital sensors: Four Thirds, launched with the Olympus E-1 in 2003. Olympus and Panasonic followed in 2008 with Micro Four Thirds, the same sensor without a mirror.
How it works
The sensor is 17.3×13 mm, with a 4:3 shape like a computer screen. Its diagonal is half that of 35mm, so the crop factor is 2. Lenses were designed to send light straight at the sensor.
Why it mattered
It was an open standard, so lenses from different makers fit. Micro Four Thirds made the first mirrorless interchangeable-lens camera, the Lumix G1, and small, light systems.
Limits and what replaced it
The smaller sensor gives more noise and less control of depth of field than APS-C or full frame. Four Thirds SLRs ended in 2010; Micro Four Thirds continues.
Today
Micro Four Thirds is still made by OM System and Panasonic, popular for compact systems and video.