The system provides a standard that, with digital cameras and lenses available from multiple manufacturers, allows for the interchange of lenses and bodies from different manufacturers. U.S. Patent 6,910,814 seems to cover the standard. Proponents describe it as an open standard, but companies may only use it under a non-disclosure agreement.
Unlike older single-lens reflex (SLR) systems, Four Thirds was designed from the start to be entirely digital. Many lenses are extensively computerized, to the point that Olympus offers firmware updates for many of them. Lens design has been tailored to the requirements of digital sensors, most notably through telecentric designs. The size of the sensor is significantly smaller than for most DSLRs and this implies that lenses, especially telephoto lenses, can be smaller. For example, a Four Thirds lens with a 300 mm focal length would cover about the same angle of view as a 600 mm focal length lens for the 35 mm film standard and is correspondingly more compact. Thus, the Four Thirds System has a crop factor (focal length multiplier) of about 2, and while this enables longer focal length for greater magnification, it does not necessarily aid the manufacture of wide-angle lenses.