Focus points are areas on a camera sensor that are sensitive to changes in lighting, detected either through a method called phase-detection or contrast-detection.

Phase-detection uses the prism inside a DSLR camera to tell whether the image is in focus. After the light from an image hits the prism, it then separates into two, which, if said image is in focus, should line up together in the camera’s viewfinder and appear sharp and focused. If the two aren’t lined up, however, the image will appear blurry and unfocused. The focus sensor in the camera controls this, and can adjust the focus accordingly to make sure the images line up. Phase-detection is typically used for action shots with fast-moving subjects, as the rate at which a camera with phase-detection focuses is often within fractions of a second.

Contrast-detection uses the image sensor itself to analyse the pixels within the image in real-time. It’s programmed to think that a subject is in focus when the contrast is the highest, pushing back the focus point of the camera lens back and forth until it reaches the desired result. Because of this, it’s a much slower method of auto-focusing and can make it very difficult to record a moving subject, which is why manufacturers often implement phase-detection in their cameras today.

The speed at which a camera focuses can also be affected by the type of focusing point used, which can be either normal or a cross-type focusing point. On higher-end models of DSLR cameras with phase-detection systems, such as the Nikon D5, there are several clusters of cross-type focusing points on the sensor, whilst lower-end cameras, such as the Canon Rebel T3i, usually have one central cross-type focus point. This means that the rate of focus in the centre of an image will be more reliable and quicker than anywhere else in the shot. On the other hand, normal focusing points tend to be much slower and less consistent, which can be difficult especially when taking action shots of a moving subject.