Autofocus (AF) automatically focuses either actively via infrared distance measurement or passively through image analysis (contrast measurement, phase detection). Active AF also works in darkness but has a limited range. Passive AF is standard and requires light and contrast. Manual focusing is often better for challenging subjects, low light, or macro photography. AF modes include single-shot and continuous AF. AF lock can affect exposure.
Category: Photography Basics > 3. Exposure & Camera Settings Article ID: 3.10 Created: May 2026
Key Points in Brief
- Autofocus (AF) automatically calculates which area the camera should focus on—without manual input
- There are two basic methods: active AF (infrared distance measurement) and passive AF (image analysis / phase detection)
- AF does not always work flawlessly—manual focus is superior for creative shots, low light, or complex subjects
Explanation
What Does Autofocus Do?
When focusing, the distance at which a plane is sharply depicted is set. The camera moves lenses inside the lens until the chosen subject is sharply focused on the sensor. The autofocus takes over this process automatically—it analyzes the image and focuses on the detected main subject.
Active Autofocus (Infrared)
Active AF was mainly common in older cameras. The camera emits an infrared beam that is reflected by the subject. Based on the reflection time or angle, the camera calculates the distance and focuses accordingly.
Advantages:
- Works in complete darkness (infrared is independent of visible light)
- Fast and simple
Disadvantages:
- Limited range (approx. 5 meters)
- Can be fooled by glass panes (glass reflects the infrared beam)
- Focuses only on a single object, no scene analysis
Passive Autofocus (Image Analysis)
Passive AF is now standard in all digital cameras. It analyzes the image itself and searches for maximum sharpness and contrast. There are two methods:
Edge Contrast Detection (Contrast Detection)
- The camera shifts the lenses step-by-step and analyzes when contrast at edges is highest
- Typical for compact cameras and live view mode
- Slower but precise
Phase Detection
- Special AF sensors (line and cross sensors) measure the image and calculate directly how far and in which direction the lenses must be moved
- Typical for DSLRs in optical viewfinder mode
- Faster and more direct than contrast detection
Advantages of passive AF:
- Unlimited range
- Works for all subjects
- No infrared source needed
Disadvantages:
- Requires sufficient light and contrast
- Limited or unusable in darkness → many cameras offer AF assist light (short red or infrared flashes for focusing)
The Two AF Modes
Single-Shot AF (One-Shot / Single AF)
- Camera focuses once when the shutter button is half-pressed
- Focus remains locked until the shutter button is released
- Ideal for static subjects
Servo AF / Continuous AF (AI Servo / CAF / C-AF)
- Camera continuously recalculates focus even while the shutter button is half-pressed
- For fast-moving subjects, focus is continuously tracked—many cameras estimate movement direction for faster response
- Ideal for: sports, animals, vehicles
- Often must be activated separately in the menu
Limits of Autofocus – When Is Manual Focus Better?
| Situation | Problem | Solution |
|---|---|---|
| Subject off-center | AF focuses on background | AF lock (half-press) + recompose, or manual focus |
| Objects in foreground (fence, branches, glass) | AF selects foreground object instead of main subject | Manual focus or manually select AF point |
| Multiple subjects at different distances | AF selects only one focus point | Manual focus on optimal sharpness plane |
| Low light / low contrast scene | AF "hunts" without locking focus | Activate AF assist light or focus manually |
| Macro photography | Minimal depth of field, AF cannot find stable plane | Manual focus, physically move the camera |
Practical Tip
When using AF lock (half-press shutter to lock focus, then recompose), always keep in mind: Exposure is usually locked simultaneously with focus—this can cause problems if brightness differs significantly between the focus point and the final composition. In such cases, it’s better to separate the AF lock button (AE-L/AF-L) from the shutter button if your camera offers this option.