page_title: Canon EF lens mount
section_title: Stepping motor
hierarchical_section_title: Canon EF lens mount / Related technologies / Stepping motor
caption_attribution_description: English: Canon EF 40mm f/2.8 STM Pancake lens
context_page_description: The EF lens mount is the standard lens mount on the Canon EOS family of SLR film and digital cameras. EF stands for "Electro-Focus": automatic focusing on EF lenses is handled by a dedicated electric motor built into the lens. Mechanically, it is a bayonet-style mount, and all communication between camera and lens takes place through electrical contacts; there are no mechanical levers or plungers. The mount was first introduced in 1987.
  In 2003, Canon introduced the EF-S lens mount, a derivative of the EF mount that is strictly for digital EOS cameras with APS-C sensors released after 2003. EF lenses can be mounted on EF-S bodies but EF-S lenses cannot be mounted on EF bodies. In October 2012, Canon introduced the EF-M lens mount, a derivative designed exclusively for mirrorless interchangeable-lens cameras with APS-C sensors. EF and EF-S lenses can be mounted on EF-M bodies via the optional Mount Adapter EF-EOS M.
  Canon claims to have produced its 100-millionth EF-series interchangeable lens on 22 April 2014.
context_section_description: Canon announced Stepping motor (STM) lenses first in June 2012, alongside the EOS 650D/Rebel T4i/Kiss X6i.
  Canon stated that this technology allows smooth and silent autofocus, and with compatible bodies (the first of which is the 650D) will provide continuous autofocus in live view and video. Unlike USM, STM lenses use focus-by-wire to enable full-time manual mode. Two main disadvantages are linked to focus-by-wire: First, the need to computationally process the input before the intended action is executed leads to a sometimes perceptible lag. Second, using the motor requires power, so when an STM lens is not connected to a camera or the camera is switched off, changing the focus is impossible.
  All EF-M lenses introduced to date—the EF-M 22mm pancake prime and EF-M 18–55mm zoom introduced with the EOS-M, plus the later 11–22mm, 15–45mm, 18–150mm, and 55–200mm zooms, and the 28mm macro lens—feature STM technology.
  All stepping-motor lenses are marked with the letters "STM" on the front of the lens as part of the model designation.
caption: Canon EF 40mm f/2.8 STM pancake lens
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