Press the shutter button halfway and the lens whirs. The camera has found focus. Autofocus is the system that adjusts the lens automatically, using sensors and algorithms to detect contrast or phase differences. It has become so fast and accurate that manual focus is now a niche skill, though some photographers still prefer it for precision work.

There are two main technologies. Contrast-detection autofocus moves the lens back and forth until it finds the point of maximum contrast. It is accurate but can hunt in low light. Phase-detection autofocus splits the incoming light and measures the distance between the split images. It is faster and better at tracking moving subjects. Most modern cameras use hybrid systems that combine both. Autofocus modes vary: single-shot for still subjects, continuous for moving ones, and face or eye detection for portraits. The number of focus points has grown from a handful to hundreds, covering most of the frame.

Autofocus modes

• Single-shot. Locks focus once and holds it.
• Continuous. Tracks a moving subject.
• Face detection. Finds and focuses on faces.
• Eye detection. Locks focus on a subject's eye.

Autofocus fails in low light, low contrast, and when shooting through glass. Knowing when to switch to manual is part of the craft.