Canon Inc. has produced seven different 24mm lenses for its Canon EF and EF-S lens mounts. Three have been discontinued after updated replacements were announced.
EF 24mm f/1.4L USM, introduced December 1997. (discontinued in 2008, replaced by EF 24mm f/1.4L II USM)
EF 24mm f/1.4L II USM, introduced December 2008.
EF 24mm f/2.8, introduced November 1988. (discontinued in 2012, replaced by EF 24 f/2.8 IS USM)
EF 24mm f/2.8 IS USM, announced February 2012, available since June 2012.
EF-S 24mm f/2.8 STM, announced September 2014. Unlike the other 24mm lenses, it will mount only on bodies that support the EF-S mount. It will mount on all current Canon bodies with APS-C sensors, as well as older APS-C bodies dating to the 2003 introduction of the EF-S mount (in other words, the EOS 10D and older bodies are not compatible). It will not mount on any body with a full-frame or APS-H sensor. As it is designed for Canon APS-C bodies, its field of view is equivalent to a 38mm lens on a full-frame sensor, and operates similar to the Canon EF 40mm f/2.8 STMpancake lens. Compared to the EF 24 f/2.8 IS USM, the EF-S lens loses IS, but lists for about a fourth of the price. The STM offers advantages for video shooting over USM, specifically quieter autofocus.