Canon EF Lens Mount - Related Technologies

Related Technologies

With the release of the EOS 300D Canon introduced a variation on the standard EF lens mount called EF-S. The "S" stands for "Short Back Focus". EF-S uses the standard EF bayonet mount, but with minor physical alterations which prevent EF-S lenses from being mounted on bodies which do not support them.

There are a couple of benefits to EF-S lenses, both related to the smaller (1.6x or APS-C) sensor size. One is that since a lens designed for a smaller sensor need only project an image circle large enough to cover the small sensor, the lens itself can be smaller; it can therefore also be lighter and have lower materials costs, since the lens elements, made of relatively heavy and expensive optical glass, will be smaller than in a comparable full-frame lens. Such a lens, if used on a body with a larger sensor, would leave the outer portions of the sensor outside its image circle, and therefore they would be black, but since EF-S lenses will not physically mount on incompatible bodies, this problem is avoided.

The second benefit is that, since a body with a smaller sensor can use a smaller mirror, the rear element of the lens can extend somewhat into the body without danger of being struck by the mirror. Particularly for a wide-angle lens, this gives the lens designers more freedom in designing the lens' optical formula.

The release of the Canon EOS M, the company's first MILC, saw the introduction of another variation on the standard EF mount, called EF-M (with the "M" presumably standing for "mirrorless"). Because Canon reduced the flange focal distance from that of its EOS DSLR line, the mount was altered so that EF and EF-S lenses cannot mount directly on EF-M bodies. Canon supplies an optional adapter that allows both EF and EF-S lenses to be mounted on EF-M bodies.