fourthirds

Four Thirds is a digital camera standard for interchangeable-lens systems, developed jointly by Olympus and Kodak and introduced in the early 2000s to create a compact, high-performance DSLR platform. It defines a specific sensor size, aspect ratio, and lens mount intended to optimize image quality and body design for professional and enthusiast use.

The Four Thirds system uses a four-thirds sensor with a 4:3 aspect ratio, typically around 22.5 by 15 millimeters in size. The diagonal is about 27 millimeters, and the format is commonly described as having a crop factor near 2 compared with 35mm full-frame. The mount, known as the Four Thirds mount, has a flange focal distance of 44 millimeters, enabling a range of lenses designed to cover the sensor area efficiently. Lenses and bodies in the system communicate electronically to support autofocus, metering, and other functions.

Early adoption produced a broad line of Olympus Zuiko Digital lenses and several bodies designed for professional and advanced amateur use. Kodak contributed to the ecosystem during the development phase, together with other participants in the Four Thirds Promoters Group. The format was praised for its image quality and robust, weather-sealed bodies, but market momentum shifted with the rise of mirrorless cameras.

From 2008 onward, Olympus and Panasonic introduced Micro Four Thirds, a smaller, mirrorless evolution of the concept that led to a rapid shift away from the original Four Thirds DSLRs. The Four Thirds format gradually declined in new products, and Olympus later deprecated the system in favor of Micro Four Thirds. Today, Four Thirds remains a part of camera history, with some legacy lenses still in use and a limited secondary market.