Autofocus

Autofocus (AF) works on the principle of light reflection from objects. Reflected light is received by the camera’s CCD sensor. After computer‑based processing, an electric focusing actuator is driven to complete focusing. Autofocus is mainly divided into two categories: active autofocus and passive autofocus.

Classification of Autofocus

1. Active AutofocusAn infrared or ultrasonic generator on the camera emits infrared light or ultrasonic waves toward the subject. The camera’s receiver captures the reflected signal to perform focusing. Its optical principle resembles triangulation ranging. One variant, the energy‑detection method, is used in low‑end consumer cameras and widely adopted in viewfinder cameras.

Active autofocus struggles with slanted surfaces and smooth reflective surfaces, as well as bright distant subjects. Emitted signals may reflect away or fail to reach the target. Since the camera actively emits light or sound waves, active AF can focus under low‑contrast and low‑light conditions. It works for fine‑detail subjects and moving objects.

Drawbacks: Focusing fails when the subject absorbs light or sound waves. Reflection from glass also prevents accurate focusing through glass panels.

1. Passive AutofocusPassive autofocus directly receives and analyzes reflected light originating from the scene itself.

Advantages: No built‑in emission system, hence low power consumption and compact hardware design. It delivers reliable performance for sufficiently bright subjects, backlit scenes, distant bright objects, and can focus through glass.

Disadvantages: Poor performance on fine‑line subjects, low‑contrast and low‑light environments, moving subjects, polarized‑light subjects, black objects and mirror‑like surfaces.

Active and passive autofocus each have strengths and weaknesses. Most DSLR cameras support both modes for complementary use and automatic switching to maximize performance. DSLRs predominantly rely on passive autofocus, which is limited by maximum aperture. Autofocus becomes difficult when aperture is smaller than F8.

For this reason, most DSLRs are equipped with an autofocus‑aid‑light emitter, projecting infrared striped beams to assist focusing on subjects of different textures. The auxiliary light remains inactive under sufficient ambient brightness.

Operating note: The active‑mode emission window sits on the right‑hand side of the camera body. Users must avoid covering it with hands, otherwise focusing will fail. Professional camera bodies do not have a built‑in AF‑aid‑light emitter. Auxiliary light for active autofocus is provided by an external flash unit.

Characteristics of Autofocus

Autofocus features three core capabilities:

1. Automatically identify the shooting subject via certain algorithms;
2. Measure the distance between the subject and the camera’s image sensor;
3. Drive a motor to move the lens focusing group to the corresponding distance position.

Autofocus Principles

Fundamentally, autofocus falls into two categories: ranging‑based autofocus that measures object‑to‑lens distance, and focus‑detection‑based autofocus that evaluates image sharpness on the focal plane.

1. Ranging‑Based Autofocus

Main implementations include infrared ranging and ultrasonic ranging.

• Infrared Ranging: The camera actively transmits infrared light for distance measurement. Geometric relationships formed by infrared LEDs are used to calculate focus distance.
• Ultrasonic Ranging: Distance is calculated based on the propagation time of ultrasonic waves between camera and subject. The camera is fitted with ultrasonic transmitter and receiver. Continuous ultrasonic pulses travel to the subject, bounce back and are detected. Integrated circuits compute focus distance from round‑trip time.

Infrared and ultrasonic autofocus rely on actively transmitted light or sound waves and are collectively known as active autofocus.

2. Focus‑Detection‑Based Autofocus

Two primary approaches: contrast‑detection autofocus and phase‑detection autofocus.

a. Contrast‑Detection AutofocusFocus status is judged by analyzing image edge contours. Sharp edges produce high luminance gradient and high contrast between subject and background. Defocused images feature blurred edges with reduced gradient and contrast; greater defocus yields lower contrast.

Two photodetectors are placed at equal distances in front of and behind the CCD plane. Scene light is split and projected onto both detectors, which output respective contrast values. When the absolute difference between the two contrast outputs reaches minimum, the focal plane lies midway between the detectors, closely aligned with the CCD imaging surface, and focusing completes.

b. Phase‑Detection AutofocusFocus is determined by measuring image offset. A grid plate of alternating transparent and opaque parallel lines is positioned at the CCD sensor plane. Two photodetectors are placed symmetrically about the optical axis behind the grid plate. The grid plate vibrates perpendicularly to the optical axis.

When the focal plane coincides with the grid plate, light passing through slits arrives simultaneously at both photodetectors. Under defocus conditions, light beams reach the two detectors sequentially, generating a phase difference between output signals. Circuit processing interprets this phase difference to actuate mechanical components and adjust lens position until the focal plane matches the grid‑plate plane.

Limitations of Different Autofocus Methods

Each autofocus technique has inherent constraints. Infrared and ultrasonic ranging may malfunction or produce inaccurate results if targets strongly absorb infrared or ultrasonic energy. Contrast‑detection autofocus is sensitive to lighting conditions; focusing becomes difficult or fails under dim light or minimal brightness difference between subject and background.

Autofocus Modes for DSLR Cameras

DSLR cameras generally offer three autofocus modes:

1. Single‑Shot Autofocus
2. Continuous Autofocus
3. Automatic‑Switch / AI‑Servo / Closest‑Subject‑Priority Dynamic Autofocus

Single‑Shot AutofocusActivated by half‑pressing the shutter button. Focusing continues until the processor confirms sharp focus. Once focus is achieved, fully pressing the shutter captures the image and the AF system stops.

If the subject moves after focus confirmation but before full shutter release, the final image may turn blurred. This mode is optimized for static subjects such as landscapes, macro shots and group portraits. Focus locks once acquired; users can recompose the frame while keeping the shutter half‑pressed.

Continuous AutofocusDeveloped to track moving subjects. Unlike single‑shot AF, continuous autofocus keeps running even after sharp focus is detected; focus is not locked. As the subject moves, the AF system dynamically drives lens adjustments to maintain sharpness. The AF frame must stay aligned with the subject during shooting.

Suitable for sports events, press conferences, wildlife and other dynamic scenarios. Combined with high‑speed burst shooting on DSLRs, it captures sequences of action shots.

Automatic‑Switch / AI‑Servo / Closest‑Subject‑Priority Dynamic Autofocus (Intelligent AF)

While single‑shot and continuous AF cover most shooting scenarios, continuous AF consumes considerable power and struggles with subjects switching between static and moving states. Intelligent autofocus combines both modes to address these challenges.

The DSLR continuously samples the AF area and feeds data to the processor. It automatically selects single‑shot AF for stationary subjects and continuous AF for moving subjects. Users only need to operate the shutter button.

Note: Single‑shot and continuous AF are standard terminology across most DSLR manufacturers. The third mode uses different brand‑specific names yet shares similar operating logic: autofocus prioritizes the focus point nearest the main subject. More AF points raise the probability of accurate subject focusing.

• Auto‑switch AF: mainly used by Konica‑Minolta
• AI‑Servo AF: Canon product feature
• Closest‑subject‑priority dynamic AF: featured on high‑end Nikon DSLRs

Types of Autofocus Lenses

Optically, AF lenses share identical structures with manual‑focus lenses, while mechanical designs differ fundamentally. AF‑lens design centers on power transmission for autofocus actuation. Based on power‑source location, AF lenses fall into two categories:

1. Body‑Driven Lenses: No built‑in motor inside the lens; used by Minolta, Nikon, Pentax and others.
2. Lens‑Driven Lenses: Integrated AF motor within the lens; represented by Canon EF‑series lenses.

AF zoom lenses are further split into manual‑zoom and power‑zoom types according to zoom actuation. Power zoom was widely implemented on compact AF cameras long ago, while power‑zoom for interchangeable AF lenses became available in recent years.

Another distinction between AF and manual‑focus lenses is the set of electrical contacts for bidirectional data communication with the camera body. A ROM chip inside the lens stores parameters including focal length, maximum / minimum aperture and minimum focus distance. The camera configures operating modes accordingly, for instance selecting program lines for program auto‑exposure based on focal length.

Contact counts vary among manufacturers, typically ranging from 5 to 8 pins. Most data‑communication contacts total 5 or 6 pins. Additional contacts deliver electrical power to in‑lens motors (AF motor, zoom motor, aperture‑control motor).

Body‑Driven AF Lenses

Driving power originates from the DSLR body. A transmission mechanism inside the lens connects to the manual‑focus ring. When the camera‑body MF/AF selector is switched to AF mode, the camera‑body AF‑motor drive shaft extends and couples with the lens transmission gear to deliver torque. The manual‑focus ring rotates during autofocus operation.

Switching to MF mode retracts the camera‑body drive shaft and disengages gearing. Users turn the focus ring manually as on conventional manual‑focus lenses.

Body‑driven AF lenses use gear trains to rotate the focus ring. The focus ring spins during autofocus and must not be touched in AF mode. To ensure smooth power transmission, the focus‑ring mechanical assembly exhibits slightly more play compared with pure manual‑focus lenses.

Lens‑Driven AF Lenses

Lens‑driven AF systems receive focus control signals and power supply from the camera body. All mechanical movements take place inside the lens with no mechanical coupling between lens and camera body.

Industrial Autofocus Lenses

As described above, autofocus relies on image‑processor analysis to evaluate focus sharpness. Consumer cameras incorporate powerful onboard image processors to complete autofocus internally.

Industrial cameras differ. Usually only motors are integrated into lenses, such as motorized three‑variable lenses (variable focal length, variable aperture, variable focus). Full autofocus requires external image‑processing systems to compute focus status and issue control commands to drive motors.

Most motorized three‑variable industrial lenses possess the hardware capability for autofocus. Actual autofocus performance ultimately depends on the capability of supporting software algorithms.