lens and pressing the nearby release button with the tip of thumb. The one-handed skillful operation of the EF mount allows changing lenses in handheld photography, since the other hand is freed to hold the camera body. When the EF mount was introduced in 1987, it had the largest mount diameter (54 mm internal) among all 35 mm SLR cameras. The EF series includes over eighty lenses, encompassing focal lengths from 8 to 1200 mm. Many EF lenses include such features as Canon's ultrasonic motor (USM) drive, an image stabilization system (IS), diffractive optics (DO) and, particularly for L-series lenses, fluorite and aspherical lens elements. Versatility Its large diameter and relatively short flange focal distance of 44.0 mm allows mechanical adaptation of EF camera bodies to many types of non-EF lenses. It is possible to mount lenses using the Nikon F mount, Olympus OM, Leica R and universal M42 lens mounts (among others) by the use of a mechanical adapter without electronic control of the aperture or autofocus. In contrast, parfocal adaptation of EF lenses to non-EF camera bodies is not possible with only a mechanical adapter that does not contain optical elements. EF mount lenses are somewhat compatible with newer Canon bodies, though the reverse is not true: EF-S lens mount cameras can mount EF lenses without an adapter EF-M lens mount cameras can mount EF lenses with an EF-EOS M adapter RF mount cameras can mount EF lenses with a variety of adapters Lenses for the earlier Canon FD lens mount are not usable for general photography on an EF mount cameras, unless adapters with optical elements are used because they are made for a flange focal distance of only 42.0 mm. Most of these lenses require autofocus and aperture motors inside the body which isn't available in EOS bodies. Infinity focus would be lost with an adapter which lacks optical elements. The Canon FD-EOS adapter is rare and is only usable with certain FD telephoto lenses. With a manual connection, the aperture and focus controls of the lens cannot be controlled or read from the camera; the lens must be focused manually. Since the only possible metering is through-the-lens, the lens must be manually stopped down to accurately meter at anything less than full aperture. (This is called stop-down metering.) Third-party lenses Compatible third-party lenses with the EF lens mount are manufactured by Yongnuo, Samyang, Schneider, Sigma,