Autofocus (AF) is a camera system that automatically adjusts the lens to bring your subject into sharp focus. Modern autofocus technology has become remarkably fast and accurate, making it an essential tool for nearly every type of photography.
How Autofocus Works
Cameras use two primary autofocus methods:
- Phase detection: Uses pairs of sensors to measure how far the lens is from correct focus and in which direction. Very fast because it calculates the exact adjustment needed in a single reading. Found in DSLRs (via the mirror) and most modern mirrorless cameras (on-sensor phase detection pixels).
- Contrast detection: Analyzes the image on the sensor and adjusts the lens until contrast peaks, which indicates sharpest focus. More accurate but traditionally slower because the camera hunts back and forth. Common in live view and older mirrorless systems.
Many current cameras use hybrid autofocus, combining both methods for speed and accuracy across the frame.
Autofocus Modes
- Single AF (AF-S / One-Shot): The camera focuses once when you half-press the shutter. Best for stationary subjects like landscapes and posed portraits.
- Continuous AF (AF-C / AI Servo): The camera continuously adjusts focus while you hold the shutter half-pressed. Essential for moving subjects like sports and wildlife.
- Automatic AF (AF-A / AI Focus): The camera switches between single and continuous based on whether the subject is moving.
AF Point Selection
Cameras offer multiple AF points spread across the frame. You can let the camera choose which point to use (auto-area), or select a specific point manually for precise control over where focus lands. For portraits, placing a single AF point on the subject’s nearest eye ensures critical sharpness where it matters most.
Subject Detection and Tracking
Advanced AF systems include face detection, eye detection (human and animal), and subject tracking. These features identify and lock onto subjects automatically, maintaining focus as they move through the frame. Eye AF has transformed portrait and wildlife photography by delivering consistent focus on the eye without manual point placement.
When autofocus struggles in low light, low contrast, or with very fast movement, switching to manual focus or using back button focus can give you more control.