Canon EF Mount Standard Lenses
Introduced in 1987, 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.
Ultrasonic Motor Drive (USM)
Ultrasonic motor (USM) lenses appeared with the introduction of the EF 300 mm f/2.8L USM lens in 1987. Canon was the first camera maker to successfully commercialise the USM technology. EF lenses equipped with USM drives have fast, silent and precise autofocus operations, and consume less power compared to other AF drive motors.
Stepping Motor Drive (STM)
Canon announced Stepping motor (STM) lenses in June 2012. This technology allows smooth and silent autofocus, and with compatible bodies will provide continuous autofocus in live view and video.
Image Stabilizer (IS)
Canon image stabilization (IS) technology detects handheld motion and optically corrects it. It only corrects handheld motion; if the subject of the photograph is moving, IS will not stop it. It also can only stabilize so much motion, ranging from two to five stops, depending on the specific IS in the lens.
In optics, the f-number (sometimes called focal ratio, f-ratio, f-stop, or relative aperture) of an optical system is the ratio of the lens's focal length to the diameter of the entrance pupil. It is a dimensionless number that is a quantitative measure of lens speed, and an important concept in photography.
A lens with a smaller f-stop is called a "fast lens" because it delivers more light intensity (illuminance) to the focal plane, achieving the same exposure with a faster shutter speed.
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