<P> The EF - S - mount is a newer subset of the EF standard, introduced in 2003 . EF - S lenses can only be used on Canon digital cameras that use the APS - C sensor, for example the 400D (EOS Digital Rebel XTi) and the 40D . EF - S lenses can be distinguished by a white dot on the mount ring, as opposed to the red dot seen on standard EF lenses . Note that while an EF - S lens cannot be mounted on a camera that uses the EF mount, EF lenses can be mounted on cameras designed for the EF - S standard: for this reason, EF - S cameras carry both a red dot and a white dot on the mount . </P> <P> As noted above under focal length, Canon makes DSLRs with various sensor sizes, and all using the EF or EF - S lens mounts . This leads to the interesting phenomenon of the same EF lens providing different angles of view depending on which camera it is mounted on . </P> <P> Third - party lenses compatible with Canon's EF and EF - S mounts are manufactured by Sigma, Tamron, Tokina and Zeiss . The manufacturers of these lenses have reverse engineered the electronics of the EF lens mount . The use of these lenses is not supported by Canon . However, many users find these lenses to be cheaper (with the exception of Zeiss), and sometimes superior alternatives to Canon lenses . </P> <P> The Four Thirds System was created by Olympus and Kodak in 2001, and is designed exclusively for digital cameras . It is a semi-open standard that may be licensed by third parties . Currently Olympus, Leica (in cooperation with Panasonic), and Sigma are making lenses under Four Thirds System consortium licensing . </P>

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