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Canon is one of the first camera brands to include a focusing motor in the lens instead of the camera body, using the STM and USM autofocus motors. Understanding the STM vs USM autofocus system in Canon lenses will help you choose a motor that will suit your application.
STM vs USM General Overview
Both STM and USM are autofocus motor technologies that help automatically move the focusing elements in the Canon lenses rather than manually moving the elements using the focus ring. The motors are versatile and allow you to switch back and forth from autofocus to manual focus.
Both motors can be used with any Canon autofocus modes, including AI Focus AF, AI Servo AF, and One-Shot AF. However, the operational mechanism of these motors is distinct, making one more suitable under certain situations than the other.
STM is an abbreviation for stepper motor, also known as a stepping motor, which is a newer generation of autofocus motors in Canon lenses. This motor is geared to turn the focusing ring for a smooth and quiet operation. During operation, the motor draws power from the camera body rather than the camera lens.
USM is an abbreviation for ultrasonic motor, which is a professional standard autofocusing motor in Canon lenses. Typically, USM is a piezoelectric motor that uses ultrasonic vibrations during operation. The motor moves the focusing elements by amplifying the vibration of the rotor and stator contact using a resonance.
Comparing STM and USM
Although you can use both STM and USM motors in a wide range of autofocus applications, an STM motor is a typical brushless DC motor using several toothed electromagnets.
On the other hand, an USM is a piezoelectric motor using single-crystal materials such as lithium niobate and lead zirconate. These design differences make the motors have the following operational similarities and differences.
Regardless of the design differences between STM and USM systems, both the STM and USM autofocus motors are designed by Canon. Since they are integrated into a wide range of lenses that share the same specs, they have the following operational similarities.
- Both systems allow you to fine-tune your focus using the manual focus ring
- Both USM and STM systems are powered by the battery in the camera body
- Both autofocus systems can achieve accurate focus, especially when shooting in good lighting
- The systems can work in any Canon autofocus mode, such as AI Servo AF, AI Focus AF and One Shot AF
Although both STM and USM motors are versatile, and you can use them in both photography and videography, they have the following design and operational differences.
Manual Focus Coupling
Generally, even the most advanced lens comes with a manual focus ring to allow you to have better control over the focusing performance. Usually, the lens allows you to switch from autofocus to manual focus and vice versa using the AF – MF switch.
That means there is some connectivity between the autofocusing motor and the manual focus ring. Usually, a USM lens uses a mechanical coupling of the motor and the focus ring, allowing you to have a full-time manual override when you turn the ring.
The mechanical coupling also lets you focus even when the lens is off. On the other hand, STM lenses have no mechanical coupling of the focusing mechanism and the focus ring, so you have to use the AF – MF switch. This also causes STM lenses to have some lag when using manual focus.
Size and Weight
Although the size and weight of the lens primarily depend on the construction materials, focal length and whether the lens is for full-frame or crop-factor cameras, it still varies with the type of autofocus motor. Generally, USM lenses tend to be larger and heavier than their STM lens counterparts.
The operational technology of the motor affects the smoothness in which the focusing elements move during operation. Usually, this can be more noticeable in videos than in photos, as you can easily see whether the video is jerky.
Generally, the USM autofocus system tends to be jerky as it operates through ultrasonic vibrations rather than smooth linear movements. On the other hand, the STM autofocus system is smoother as the motor turns in smoother steps rather than vibrations.
Regardless of the technology the motor uses, there are moving parts that help to move the focusing elements. That means almost all autofocus systems produce some noise due to the friction of the moving parts.
However, the noise levels vary with the motor’s design quality and operational technology. Usually, the USM systems are noisier than STM systems due to their larger sizes and vibrations during operation instead of smooth movements.
Although the lens build quality doesn’t necessarily depend on the incorporated autofocus system, the USM system is regarded as a professional standard and is usually included in the high-end Canon L-series lenses.
In most cases, these lenses are constructed of high-quality materials such as metallic barrels and come with weather sealing, making them suitable for use in trying conditions such as dusty deserts or snowy environments. Typically, this means USM lenses attract a higher price tag.
On the other hand, STM lenses tend to be entry-level, with lower quality glass and barrel. In most cases, these lenses are constructed of plastic, and the glass can break or the plastic barrel crack in case of accidental falls. Most of these lenses are also not weather-resistant, making them unsuitable for harsh situations.
Main Distinguishing Factor
The main distinguishing factor between STM and USM is speed. Since USM motors use vibrations to move the elements, they are faster than the STM systems, and they can achieve an accurate focus and lock into place within milliseconds.
Usually, this makes USM lenses the best for use with AI Servo AF as they can rapidly track a subject that is constantly changing from still to motion.
When to Use STM
Since the STM system uses a typical DC stepper motor that can move the elements more smoothly and quietly, it can be the best option to use under the following situations.
- When you want to shoot a stable and smooth video without jerks
- When you are using the camera’s built-in microphone to capture the audio
- If you are a beginner and you want a budget lens
- If you are shooting handheld and you want a compact and lightweight lens
When to Use USM
Keeping in mind that the USM is a faster and more accurate system, it might be the best to use under the following circumstances.
- When capturing action or fast-moving subjects such as wildlife and birds
- When using a tripod setup and you want to improve the tripod stability with the added lens’s weight
- If you are a professional photographer looking for a system with accurate focus
- If you are shooting in low-light conditions and you want a full-time manual override so you can easily fine-tune the focus
- If you are shooting in trying conditions and you want a lens with a weather sealing
Which Autofocus Motor Is Better?
The USM autofocus motor is better as it allows you to achieve a fast and accurate focus, especially when capturing stills of a moving subject. However, the STM system might be the best choice when capturing videos due to its smooth and quiet operation.
This STM vs USM guide reveals the design and operational similarities and differences between the two systems, helping you choose the system that will suit your photography or videography application. Both systems are versatile and will allow you to use manual focus for better focus fine-tuning.