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What is AI Focus?

The Basics:

AI Focus – short for Artificial Intelligence Focus - is one of three different automatic focusing modes available on Canon EOS DSLR cameras. It combines some of the characteristics and behaviour of the other two AF methods – One Shot AF, and AI Servo AF – depending on the subject, the scene, and other photographic considerations.

A Little More Detail:

AI Focus is the least used – and probably understood - AF Mode

Most photographers will habitually use One Shot AF when photographing a subject that doesn’t move, for example, a posing portrait subject or still life. On the other hand, a photographer would use the AI Servo AF mode when taking a picture of something moving, such as a race car, a football player, or a swimmer for example. 

So where does that leave AI Focus AF? It is often said that AI Focus AF is a cross between One Shot AF and AI Servo AF; it behaves like One Shot AF until the subject moves, after which it behaves like AI Servo AF.

In reality, it’s probably best not to look at it that way, and instead, think of AI Focus AF as being an extension of One Shot AF.

In One Shot AF mode you can focus on your subject with a half-press of the shutter button (or by pressing the AF-On button).  Once focus has been attained, you will hear an audible ‘beep’, which gives you the signal to go ahead and capture the shot. You can also recompose your scene, knowing that your focus has been set. This is called AF Lock.
See One Shot AF

Initially, AI Focus AF behaves identically to One Shot AF. The camera will achieve focus and emit the confirmation beep, which, allows you to go ahead and capture the shot. However, the critical difference is that the camera will continue to monitor the distance to the subject, and if it changes, will attempt to refocus.

Because of this behaviour, it is not possible to lock focus and recompose. Instead, the photographer should get in the habit of moving the active focus point around the frame instead. This approach is actually good practice, particularly when shooting with a very shallow depth of field.

AI Focus will always assume that the subject is reasonably still.  For that reason, AI Focus AF should never be used for a subject that you know is constantly moving. In cases such as that, you should always switch to AI Servo AF, and enjoy far more effective – and customisable – focusing behaviour.