Yashica and Contax manual-focus lenses with a common bayonet mount were introduced in 1975 simultaneously on the Contax RTS and on the Yashica FX-1. The Yashica range of lenses included a premium range of lenses for the more serious photographer (termed 'ML' or 'MC' for multi-coating), while consumer SLR cameras were usually equipped with 'DSB' lenses, which had single-coated optics. The top of the line was the renowned Zeiss AE series of lenses, intended for the Contax SLR cameras, each of which displayed a T* for Zeiss' proprietary multi-coating. Some Carl Zeiss lenses were made in Germany and the rest in Japan under Zeiss license. But all of these lenses were interchangeable with any Yashica or Contax camera equipped with the C/Y bayonet lens mount.
In 1983, Kyocera acquired Yashica, and continued to manufacture Yashica ML lenses, as well as the Carl Zeiss AE T* lenses under license. The C/Y mount was slightly modified in 1985 with the new Zeiss 'MM' design lenses, which permitted the use of program and shutter-priority on the Contax 159MM and subsequent bodies. The new MM lenses are distinguished visually by the color of the f-stop markers: the highest f-stop on the MM lenses is marked in green. (On the older lenses, also called AE lenses, it is marked in white.) The newer MM lenses are still backwards-compatible with all Contax and Yashica cameras, but with older AE lenses one cannot use all available programmed and shutter-priority exposure functions on the post-1984 Contax bodies. Unfortunately, the program mode of later manual-focus Yashica (Kyocera) bodies (FX-103, 107/108/109MP) does not have full MM compatibility.
One of the more interesting adaptations of the Zeiss AE/MM lenses is to the Contax AX camera, built by Kyocera. This camera allows autofocus of the manual-focus Zeiss lenses, by actually moving the film plane (rather than the lens) back and forth. The AX can also be used with other lens mounts, via adapters.
Later Contax SLR lenses for the auto-focus Contax N series built by Kyocera use a completely different and dedicated AF lens mount, aptly called "N mount."
While every brand has its adherents, optical testing has generally shown that Zeiss AE/MM lenses were among the sharpest ever made for a production 35mm SLR. Adapters to use Zeiss T* AE/MM lenses on other cameras, most notably Canon EOS digital SLRs, have proved popular.
While not to Zeiss standards, Yashica ML and MC multi-coated lenses for the C/Y bayonet mount have also earned a reputation for high quality. Yashica already had a reputation for fine optics dating from the early 1960s with its optics supplier (later subsidiary), Tomioka Optical, and is also believed to have benefited from its licensing and manufacturing relationship with Carl Zeiss.