Autofocus (AF) is one of the most significant advancements in the history of photography. From the first autofocus camera — the Konica C35 AF in 1977 — to the first true autofocus ILC, the Minolta Maxxum 7000. Although manual focus still has its devotees — especially in genres that reward deliberation, such as macro, landscape, or vintage shooting — autofocus has become the default expectation for most modern photographers.
Whether capturing fleeting expressions, fast-moving wildlife, or cinematic video sequences, autofocus systems are engineered to keep subjects sharp, responsive, and accurately rendered. But beneath this convenience lies a complex interplay between camera-based detection systems and lens-integrated motors.
To understand how autofocus works — and why some systems excel in certain contexts while others falter—we must explore both how a camera determines focus and how the lens physically executes that command. These two components work together, and the effectiveness of one often depends on the precision of the other. Autofocus is not monolithic; it is a layered architecture of optical, electronic, and mechanical processes that differ from brand to brand, and even lens to lens.
Camera-Based Autofocus Systems
The first half of the autofocus equation occurs within the camera body. This is where the system “decides” where to focus, using various detection strategies to analyze the scene and measure sharpness.
Contrast Detection Autofocus
Contrast detection is the most intuitive and mathematically straightforward autofocus method. It operates on a principle familiar to anyone who has squinted to bring something into focus: the sharpest image is the one with the most local contrast.
In digital cameras using this method, the sensor evaluates contrast by measuring differences in brightness between adjacent pixels. As the focus changes, contrast increases until it reaches a peak. The camera then stops focusing, having determined that it has reached maximum sharpness. This process is iterative and non-predictive; the lens must move past and return to the point of sharpest contrast to confirm it.
While highly accurate in still scenarios, contrast detection suffers in speed. Because it does not know in advance which direction or how far to move the lens, it often results in “hunting,” especially in low light or with low-contrast subjects. This can lead to delays and missed focus moments in action photography. However, the precision of this method makes it valuable for static subjects, studio work, or video scenarios where accuracy takes precedence over speed.
Phase Detection Autofocus
Phase detection autofocus transforms focus into a problem of geometry. Unlike contrast detection, which evaluates sharpness after moving the lens, phase detection estimates the direction and magnitude of lens movement required before initiating any movement. This preemptive calculation makes phase detection far faster.
In DSLR systems, phase detection typically involves a separate AF module located in the camera body, using mirrors and beam splitters to divert part of the incoming light to a dedicated sensor. In mirrorless systems, phase detection pixels are embedded directly on the image sensor. This allows the camera to simultaneously capture the image and analyze focus.
The system works by comparing two versions of the image projected from opposite sides of the lens. When these two projections are in phase, the image is in focus. If they are out of phase, the system can immediately tell whether to move the lens closer or farther away, and by approximately how much. This speed and directionality make phase detection ideal for fast-moving subjects and continuous autofocus tracking.
However, in DSLR configurations, phase detection is prone to calibration errors. Since the AF sensor and image sensor are physically separate, slight misalignments—known as front- or back-focusing—can occur. Mirrorless systems with on-sensor phase detection have largely resolved this issue, merging accuracy with speed.
Hybrid Autofocus
Recognizing the complementary strengths of contrast and phase detection, many manufacturers now combine these two technologies into hybrid systems. In these arrangements, phase detection provides a quick rough estimate of focus, while contrast detection refines the result to achieve precise sharpness.
This synergy reduces hunting while preserving accuracy, and it has become the standard approach in most modern mirrorless cameras. Hybrid systems are particularly effective in video autofocus, where smooth transitions and accurate tracking are essential. The blending of these technologies also allows for better performance across a broader range of shooting scenarios, from high-speed sports to quiet interviews.
Canon Dual Pixel Autofocus
Canon’s Dual Pixel CMOS Autofocus (DPAF) is a unique (and proprietary) implementation of phase detection that relies entirely on the image sensor. Instead of dedicating select pixels for phase detection, Canon splits every pixel on the sensor into two photodiodes. These dual photodiodes each gather light from slightly different angles, allowing each pixel to act as its own miniature phase detector.
Because nearly every pixel is involved in both image creation and focus detection, DPAF enables nearly 100% AF coverage and remarkably smooth subject tracking. The system is particularly valuable in video, where its ability to follow a subject through complex movements without hunting or pulsing has earned widespread acclaim. This is why Canon C series cinema cameras were extremely popular for documentaries and reality TV. It is also highly effective in stills, especially in continuous focus scenarios involving unpredictable motion.
Canon has continued refining this technology with Dual Pixel CMOS AF II, which adds improved object recognition, eye tracking, and even animal detection. The strength of DPAF lies not just in speed or precision, but in the fluid, organic way it adjusts focus during live recording — something traditional systems have historically struggled to achieve.
Panasonic Depth from Defocus (DFD)
Panasonic, lacking on-sensor phase detection in many of its Micro Four Thirds and early full-frame models, developed an entirely different autofocus system called Depth from Defocus (DFD). This approach relies on computational modeling rather than relying solely on phase comparison or contrast evaluation.
DFD works by analyzing two out-of-focus images taken in quick succession. By comparing the blur characteristics of these frames against a built-in database of lens profiles, the camera estimates the subject distance and how much lens movement is needed. This system is faster than pure contrast detection, but it does suffer from some jitter and strange transitions when shooting video.
However, its reliance on lens-specific data means that DFD performs best with native Panasonic lenses. When used with third-party or adapted optics, results can become inconsistent. Additionally, DFD does not match the precision or tracking performance of advanced phase detection systems in fast-paced environments (which is why recent Panasonic cameras have moved to hybrid detection).
AI-Based Autofocus and Subject Recognition
Increasingly, modern autofocus systems are intertwined with artificial intelligence. These systems can detect and prioritize specific subjects—human faces, eyes, animals, vehicles—based not just on focus metrics, but on pattern recognition and predictive learning. These algorithms analyze visual data and make context-aware decisions about which part of a scene should remain sharp, even if the subject is partially blocked or moving unpredictably.
Sony, Canon, and Nikon all employ advanced subject-recognition autofocus systems in their flagship mirrorless cameras. The Sony A1, Canon R3, and Nikon Z9 use machine learning models trained on vast datasets to identify subjects by their shape, behavior, and movement patterns. These systems can distinguish between a bird in flight and a person riding a bike, dynamically adjusting the focus zone in real time.
AI autofocus makes tracking not just more accurate, but more intuitive. It allows photographers to concentrate on timing and composition, trusting that the camera will handle technical precision.
Lens-Based Autofocus Systems
While the camera determines what to focus on, the lens is responsible for executing that decision. Autofocus motors within the lens physically move internal elements to adjust the plane of focus. These motors vary widely in performance, noise, and compatibility with video.
Micromotors
Micromotors—often simple DC motors—represent the earliest generation of autofocus mechanisms. They operate using a basic geared drive, slowly rotating the focus elements toward the desired position. These motors tend to be loud, imprecise, and slow, particularly in entry-level lenses. They also typically do not support full-time manual focus override, meaning that switching to manual requires toggling a switch or risking damage to the motor.
Although cheap and serviceable for casual users, micromotors have largely been phased out in favor of more advanced alternatives.
Ultrasonic Motors (USM, SWM, HSM, SSM)
Ultrasonic motors revolutionized autofocus in the 1990s and remain a mainstay in professional lenses. These motors use high-frequency ultrasonic vibrations to drive movement. Canon’s “USM,” Nikon’s “SWM,” Sigma’s “HSM,” and Sony’s “SSM” are all iterations of this principle.
There are two main types of ultrasonic motors: ring-type and micro-type. Ring USM motors are more powerful and generally used in high-end telephoto or zoom lenses. Micro USM motors are smaller and cheaper but less efficient.
Ultrasonic motors are prized for their speed, torque, and near-silent operation. They also allow for full-time manual focus override—users can grab the focus ring at any time without switching modes. These characteristics make them especially useful for wildlife, sports, and any setting where discretion and responsiveness are critical.
Stepping Motors (STM)
Stepping motors (or “stepper motors”) move lens elements incrementally, one precise step at a time. This deliberate movement results in very smooth and quiet focus transitions, making STM lenses especially popular among video shooters.
In still photography, STM can be slightly slower than ultrasonic motors, particularly when focusing from near to far. However, for portraits, landscapes, and most general photography, the performance difference is negligible. For video, STM offers a distinct advantage by producing less audible focus noise and more organic racking transitions.
Canon was an early adopter of STM in their EF-S and EF-M lenses, followed by Nikon with its AF-P series and Sony with several E-mount designs.
Linear Motors
Linear motors depart from rotational movement entirely. Instead of turning gears or rotating screws, these motors move the focusing group directly along a straight track using electromagnetic propulsion. This method results in immediate acceleration, near-instantaneous stopping, and zero perceptible delay.
The quiet operation and high precision of linear motors make them ideal for both high-speed photography and cinematic video. Fujifilm, Sony, and Sigma now rely heavily on linear motors in their premium lenses. The Sony 70-200mm f/2.8 GM OSS II, for example, uses four XD (extreme dynamic) linear motors to drive focus groups independently — enabling some of the fastest autofocus performance ever seen in a zoom lens.
These motors also allow for better weather sealing and compact construction, which is essential in mirrorless systems where size and weight are more critical than ever.
Voice Coil Motors (VCM)
Voice coil motors are another form of linear motor that resemble the technology found in audio speakers. They move focus elements using magnetic fields generated by a coil and a fixed magnet. Because they can adjust position smoothly and quietly, they’re well suited to lenses with small focus groups, such as compact primes or macro optics.
Olympus and Sony have experimented with VCMs in select lenses, and their compactness and accuracy make them ideal for continuous AF or high-speed burst shooting.
Piezoelectric Motors
Piezoelectric motors use materials that expand or contract slightly when voltage is applied. These subtle movements can drive lens elements with great precision and minimal noise. Though less commonly seen, piezo motors offer another tool in the engineering toolkit, particularly in niche or specialty optics where space and vibration damping are crucial.
Conclusion
Autofocus is no longer just a convenience—it is a foundational aspect of how photographers interact with their subjects. From the rapid calculations of phase detection to the deliberate refinement of contrast detection, from Canon’s elegant dual-pixel design to Panasonic’s unique DFD approach, autofocus technologies are as varied as the lenses and bodies they support. On the mechanical side, the choice between micromotor, ultrasonic, stepping, linear, or voice coil motors affects everything from speed to sound to tactile experience.
As camera systems evolve, autofocus continues to be an area of fierce innovation. Understanding these systems—not just what they are, but how they work—empowers photographers to make more informed decisions about gear and technique. In a world increasingly dominated by speed, automation, and AI, clarity about the tools of focus remains essential for staying focused on the image itself.
Header photo licensed by CC-BY-SA 4.0
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