The Four-Thirds system is an innovative, open standard for digital SLR cameras that is designed to optimize their size, performance, compatibility, and extendibility. Standardization of lens mounts enables photographers to mix and match interchangeable lenses and camera bodies from different makers.
Because lenses traditionally optimized for film cameras don't always perform well with the digital camera's image sensor (the device that captures light in the form of electrical signals), the Four Thirds system introduced by Olympus and Kodak was the first digital-dedicated design. Four Thirds lenses are highly computerized and tailored to the requirements of digital cameras.
Small size sensors mean that lenses and camera bodies can also be lighter in weight and extremely compact. Zoom lenses, in particular, pack impressive specifications into petite size. The latest development, Micro Four Thirds, devised by Olympus and Panasonic for mirrorless interchangeable-lens digital cameras, reduces size even further.