Auto-focus is a feature of digital cameras that allows them to focus correctly on a subject. It enhances the quality of the photo over fixed-focus cameras and allows correct focus for close-ups (or the even closer macro shots). 

Smart phones

Nowadays, the majority of smart phones (except some entry-level devices) have adjustable focus. The lens can be moved back and forth, this changes the distance between the lens and the sensor. Used together with auto-focus technology, it means you’re able to capture sharper images without blurring on the sensor and loss of detail. Phones use passive auto-focus with contrast measurement. 

In contrast-detection auto-focus, your smart phone camera will move the lens back and forth until it finds the point of maximum contrast. This will be when the image is in focus.

Other cameras

Most cameras use the Phase-Detection auto focus. The technique is more commonly seen on DSLRs rather than smart phones or compact cameras. 

With phase-detection auto-focus, your camera analyses the light as it passes through the lens and compares light from two apertures on opposite sides of the lens. This creates two profiles of light intensity. If the image is properly focused, the two light rays will give an overlapping intensity profile.