AI Servo AF is a selectable AF mode, for when shooting through the viewfinder in an EOS DSLR
. Short for Artificial Intelligence Servo Automatic Focusing, AI Servo is Canon’s dedicated focusing mode for moving subjects. It has the ability to track and analyse movement, and focus the image based on where it predicts the subject will be at a given point in time. AI Servo AF is the preferred focusing method for Sports photographers, and any kind of photography where the subject is constantly moving.
AI Servo AF is all about capturing movement; the main problem here is that no two moving objects are the same. For example, imagine a sprinter running in a straight line directly towards you. Contrast that with a race car appearing from your left, driving past and then disappearing round the corner to your right. And then, think about a soccer player who is constantly changing direction, accelerating, and disappearing behind other players. How can the photographer adapt to those different situations, and still nail the shot?
The answer lies in the technology behind Canon’s AI Servo AF
. It’s incredibly complex, and is constantly being improved over successive camera generations.
The secret behind the effectiveness of AI Servo AF is that it does not necessarily focus on where the subject is, but based on its movement, will predict where the subject will be when you take the shot.
Crucially, focus is not locked in the same way that it is with One Shot AF – the camera will continuously focus for as long as you activate auto-focus (ie by half pressing the shutter button, or depressing the AF-On button), and will not prevent the photographer from capturing the shot. For this reason, the photographer will almost always set the camera to High-Speed Continuous shooting, whereby it will continue to take sequential shots for as long as the shutter button is held down.
Depending on the camera model, the photographer can customise the behaviour of AI Servo AF in a number of ways.
For example, you can change how quickly the camera responds to changes in acceleration, the effectiveness of subject tracking, and how the camera responds to new subjects entering the frame. Selected EOS cameras have 6 different combinations of these settings as saved pre-sets. These are known as ‘Cases’, and can be quickly changed by the photographer depending on the scene.
Additionally, the photographer can also modify the behaviour of the shutter release, depending on focusing conditions. For example, you can set the camera to only take shots once it has successfully focused in AI Servo, or you can place priority on capturing the shot, regardless of focus. This behaviour can be adapted, depending on whether it is the first or second image captured in a scene. For example, you specify that the first image is taken as soon as you press the shutter button, but that successive images in a burst sequence should only be captured once the subject is in focus.