Focusing modes

Broadly speaking there are two different focusing modes based on the subject movement and the one between these two modes. 

Single-Servo/One shot

This is the most commonly used autofocus mode. All you have to do is point the camera at the subject, half press the shutter-release button and the lens will focus automatically. The focus point will remain locked for as long as you maintain the half press on the shutter release button, enabling you to recompose the shot while keeping the selected point in sharp focus. Recomposing many a times works out to be faster than changing the focusing points.

With Focus priority set in the custom settings, compared to the release priority, the focus will remain locked as long as you keep the shutter elese button half pressed and if the subject goes out of focus, it wont click. So the shutter will release only if the subject is focuses. However, this can be overridden as well.

Continuous/AI Servo

This is more suitable for moving subjects, such as wildlife or sports. The main difference between this and one-shot autofocus is that the lens continuously focuses on your subject in this case. The autofocus system tracks the subject to ensure that it’s sharp in the captured image.

By default, continuous mode enables you to take a shot whether focus is achieved or not. There are limitations, such as in fast sports, such as a racing car coming towards the photographer at a higher speed than the autofocus can keep up with. In such situations, it’s best to pre-focus in one-shot mode.

AI Focus

In this ‘intelligent’ autofocus mode, often referred to as AI Focus, the camera’s autofocus system automatically senses whether an object is stationary or moving, and then switches between one-shot and continuous modes.In general it stays at One shot and if the subject moves, it becomes the continuous mode.