Four Thirds Lenses
17.3 x 13 mm image area · 22 interactive lens diagrams
Olympus and Panasonic Four Thirds / Micro Four Thirds stills and video systems | 17.3 x 13 mm sensor with a 21.64 mm diagonal
Four Thirds was designed as a digital camera format rather than inherited from a 35 mm film gate. Its 17.3 x 13 mm sensor uses a 4:3 aspect ratio and a diagonal almost exactly half that of full-frame, giving the format its familiar 2x angle-of-view crop factor. The smaller image circle lets lenses be more compact than equivalent 35 mm designs, especially in telephoto and standard zoom ranges where coverage and glass diameter dominate the package.
The format also changes photographic tradeoffs. For the same field of view and f-number, depth of field is deeper than full-frame, and designers use shorter focal lengths to achieve familiar angles of view. Four Thirds DSLR lenses and Micro Four Thirds mirrorless lenses share the capture format, but mirrorless bodies remove the SLR mirror box and can use shorter back-focus layouts. The catalog's Four Thirds entries show how compact digital-era lens systems can be when sensor size, mount geometry, and optical design are planned together.