Four Thirds System

The Four Thirds system is a standard created by Olympus and Kodak for digital single-lens reflex camera (DSLR) design and development.

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 SLR systems, Four Thirds has been designed from the ground up 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 crop factor (focal length multiplier) of about 2, and while this enables longer focal length for greater magnification, it doesn't aid the manufacture of wide angle lenses.

The image sensor format, generally intermediate between those of larger SLRs and smaller compact cameras, generally gives levels of cost, performance and convenience intermediate between those two classes.