Autofocus: the basics

In short: Autofocus adjusts the lens automatically so that the chosen subject appears sharp. If you understand the modes and focus points, you will get noticeably more sharp pictures.

In more detail: Cameras measure sharpness mainly through phase detection or contrast detection, and many modern models combine both directly on the sensor. Many devices also detect faces, eyes or animals automatically.

In detail

• Phase detection: From the offset between two beams of light the camera works out in which direction and how far it must focus; this is fast and good for movement.
• Contrast detection: The lens moves back and forth until image contrast is at its highest; this is accurate but often somewhat slower.
• Single autofocus: In AF-S mode (called One Shot by some manufacturers) the camera focuses once and holds that focus, ideal for still subjects.
• Continuous autofocus: In AF-C mode (also AI Servo) the camera keeps adjusting focus to follow a moving subject.
• Focus points: A single focus point gives full control, while large areas or automatic selection help with fast, unpredictable subjects.
• Subject detection: Eye and face detection is especially helpful for portraits, because focus should sit on the nearest eye.
• Difficult situations: In low light, low contrast, with reflections or with a fence in front of the subject, autofocus can become unreliable.
• Back-button focus: Some photographers assign autofocus to a button on the back to separate focusing from releasing the shutter.
• Manual focus: Aids such as focus peaking and image magnification make focusing by hand easier, for example in macro or night photography (blende-und-schaerfentiefe, portraetfotografie-tipps, makrofotografie-grundlagen).