Camera's autofocus system requires a motor to physically move the lens to bring the subject into focus.
Unlike most camera makers Canon chose to position the autofocus motor inside the lens barrel rather than the camera body.
This means autofocus motor can be tailored to the requirements of each lens. Also, the focus motor in the lens is often faster and quieter than the camera's in-body focus motor.
Which means that we can customize the motor as per the lens. A bigger motor for telephoto and a compact one for prime lens.
However if the autofocus motor being only in the camera body don’t have this flexibility
Lets understand the different type of motor technologies Canon employ in their lenses:
Electromagnetic motor drives:
Such motors contain tiny wound coils of wire and rely on electromagnetic principles to turn a shaft. Little cogwheels and gears are then used to translate this rotational motion into the movement needed to adjust lens focus.
They’re somewhat noisy - electric buzzing and grinding of gears - and not terribly fast Thus result in Slow focusing and a bit noisy.
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Ultrasonic Motor (USM)
The Ultrasonic Motor (USM) is a new type of motor which found its first application as a camera lens motor in Canon EF lenses. The ring USM which made its debut in 1987 in the EF 300mm f/2.8L USM amazed the world with its silent, super-fast auto focusing performance EF lenses equipped with USM drives have fast, silent and precise autofocus operations, and consume less power compared to other AF drive motors.
There are two types of Ultra Sonic Motors:
A. Ring ultrasonic motors
B. Micro ultrasonic motors
Unlike the other two types of motors that rely on magnetic coils, the USMs operate with high-frequency vibrations. These motors are very fast and virtually silent.Customer must carefully examine the specifications of the individual lens to determine which type of motor is used.
When purchasing a Canon lens, both types of motors will be labeled as a USM.
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A. Ring Ultrasonic Motor
The ring ultrasonic motor consists of two metal rings which vibrate at a high frequency. The sound waves transfer energy without any moving parts. These lenses focus very quickly, precisely, with no sound, and are capable of being switched to full time manual mode.
B. Micro Ultrasonic Motor
The micromotor USM is a less expensive version of the ring USM. This type of motor replaces the standard magnetic motor but use the same type of gear train. The micromotor USM is still a very quiet lens, but a customer is unable to override the autofocus to use manual focus, as can be done with a ring USM.
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Stepping motor (STM)
- STM stands for "stepping motor".
- An STM is a motor that is driven in synchronization with pulse power, and rotates 1 step per electrical pulse. Therefore, it is sometimes referred to as a pulse motor.
- STM features its exceptional responsiveness and controllability. Also, the motor’s simple mechanical structure contributes to its smooth lens drive and reduction in the size of the lens barrel.
All stepping-motor lenses are marked with the letters "STM" on the front of the lens as part of the model designation
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Drive mechanism explanation
STM Focusing Mechanism
According to the lens type, one of two STM mechanisms may be used.
Lenses: EF-S18-135/3.5-5.6 IS STM
The rotary drive is a simple, direct drive mechanism that moves the lens barrel. The result is remarkably smooth and silent servo AF for movie shooting when mounted on EOS 650D.
Lenses: EF40/2.8 STM
Helical gears with angled edges in the gear unit realize more silent and smooth movie servo than a DC motor when mounted on EOS 650D.
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Different manufacturers regarding autofocus motors within the lens
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