Lens Speed
You will see mentions of the speed of a lens, as in a ‘fast’ lens. This refers to the maximum, or widest aperture of the lens, and is relative to the type and focal length.
For example, the EF24mm f/2.8 has an average speed, but the EF24mm f/1.4L II is fast. The EF50mm f/1.8 II has an average speed, while the EF50mm f/1.4 USM lens is fast. With super-telephoto lenses (300mm or more), an aperture of f/2.8 is fast.
It is more difficult to design zoom lenses with really wide apertures, as they usually have more lens elements inside. Here, anything with a maximum aperture wider than f/3.5 is considered fast. The EF16-35mm f2.8L II USM and EF24-70mm f2.8L II USM lenses are examples of fast zooms.
If you are familiar with the earlier Canon FD lenses, or many other camera lenses, you will notice that EF lenses do not have a ring for setting the aperture. The aperture is controlled from the camera body. An electrical pulse is sent to the lens via the gold-plated contacts in the lens mount. This pulse is received by the electromagnetic diaphragm (EMD) where a stepping motor adjusts the aperture to the value set on the camera.
A second motor inside the lens looks after the automatic focusing. One of several different motors is used, depending on the type of lens and its target market. Top of the range is the ring ultrasonic motor (USM), used in all the L-series and other high-performance lenses.
In this unusual type of motor, pioneered by Canon, the rotational force comes from ultrasonic vibrational energy, rather than the electromagnetic force used by conventional motors. From this was developed the Micro USM and Micro USM II motors, which are found in some of the recent consumer lenses.
Some of the earlier consumer EF lenses use an arc form drive (AFD) motor. This is basically a conventional small motor unit, but shaped to fit inside the curved barrel of a lens. A tiny micro motor has replaced the AFD in some later lenses.
The USMs are faster and quieter than the AFD or micro motors. They are also more expensive.