Getting to Know Canon's AI Servo

by Ron Donson

(This tip was submitted by a user.)

If you're a Canon photographer and you shoot moving subjects then you're probably aware of AI Servo. This article is a brief introduction to it with a few tips that might help you use it more effectively.

What is AI Servo?

AI Servo is Canon's predictive autofocus system. The name is derived from the use of Artificial Intelligence used to predict the speed and distance of the moving subject. It greatly increases your chance of getting a sharp image when your target is moving.

How does AI Servo work?

There are two key concepts at work.

• Predictive autofocusing
  
  As you might imagine, predictive autofocusing happens whenever AI Servo is on and there's a readable subject with predictable movement.
  
  
• Shutter release control
  
  Shutter release timing is controlled by the photographer for single shots and the first shot in a burst. Shutter release timing is controlled by the camera for subsequent shots in the burst.

You can see AI Servo in action. With the shutter button pressed halfway track the motion of a moving subject with the active focusing point. You should be able to see the lens continuously autofocusing.

In AI Servo, you can tell whether the AF system is tracking the subject by observing the in-focus indicator in the viewfinder below the picture area.


Figure 1 from Canon's "EOS-1D Mark II manual"

If you *can't* see the focus confirmation light, the system is tracking. If it is blinking rapidly, the system is telling you that it is not tracking. This is slightly different from the camera's behavior in One Shot AF. In One Shot, the in-focus indicator blinks when the subject can't be focused, but it lights up when focus has locked, as opposed to no in-focus indicator appearing when the subject is being successfully tracked in AI Servo AF.

Techie gorp

The EOS-1D Mark II is the first Canon to have two CPUs taking care of the functions of focusing and driving the lens.

Dividing tasks means decisions can be made simultaneously rather than in sequence. For example, in One-Shot AF mode, AF processing, SI display (i.e., illumination of the active focusing point in the viewfinder) and aperture stop-down are executed in parallel with lens driving and mirror flip-up. In AI Servo AF/Predictive AF, statistical prediction using the focusing data from previous focusing operations is incorporated. The number of focusing operations per unit time is twice as many as the 1D's. With shorter time intervals and more repetitive focusing operations, the predictive AF control works effectively from the first shot even with subjects moving erratically. Should the subject's movement change just before shutter release, the shorter focusing operation interval means the predictive AF control has a higher probability of catching it.

The AF CPU is a 33 MHz, 32-bit RISC (reduced instruction set) microcomputer that handles area AF detection and auto AF point selection. The camera CPU is a 32 MHz, 32-bit RISC microcomputer that controls lens communications, lens driving control and predictive AF statistical calculations. As a result, all the processing is faster than with the EOS 1D's AF control. One-Shot AF speed is faster and AI SERVO AF focusing precision is higher. - Chuck Westfall


Figure 2 from "Getting the Most from your EOS-1 Class Digital SLR"

Developing AI Servo shooting techniques

The first rule of thumb is that if you want that first shot in AI Servo to be sharp then you should be tracking the subject for at least 1-2 seconds prior to shutter release.

Focus Points

• Single point AF is generally better for team sports and situations where you're trying to select a subject from a group.
• AFPS (Automatic Focus Point Selection) is generally better for individual sports and wildlife such as birds and running animals where your subjects are by themselves.

Custom Functions

1. C.Fn. 4-3 This setting is preferred by many sports shooters. It provides several advantages.
   • Focus is now engaged with the * button on the back of the camera with your thumb instead of depressing the shutter half-way.
   • This isolates exposure from focus. Exposure is now calculated when the shutter is depressed.
   • This reduces the number of accidental shutter firings.
   • The camera is more responsive in focusing.
     
     
2. C.Fn. 17 This setting expands the AF activation area.
   • 0 - default - only the selected AF point or points are active
   • 1 - expands the active focus area by a radius of 1 around the manually selected AF point. Up to 7 AF sensors around the manually selected AF point can be activated. This can be useful in low contrast situations.
   • 2 - expands the active focus points up to 13 AF sensors around the manually selected AF point. This is camera and lens dependent.
   
   
   The default (0) is the fastest performing but 1 and 2 can increase the chances of finding a small subject or one in low contrast conditions.

Conclusion

Well, that's the basics of AI Servo. It's a very powerful tool for photographers. Hopefully this will help you get started in getting more keepers on those moving subjects. For a deeper understanding check out the information sources at the bottom of this page.

In Practice

Note from Adrienne: Anake tried out Ron's tip, and was very happy with the results. This is what he had to say (edited slightly with Anake's approval): "When I visited this place, I thought about trying RonD's tip. So, I switched to CFN 4 and CFN 17 with AI servo and practiced for a few rounds. It worked like magic and I fully understood why sport photographers advise this setup for the Mark 2. Now, I preset the setup into my personal Custom function. If you want to use my images for demo in RonD's tip, I will be happy to share them with SF."

And here's one of Anake's great action photos, taken using these settings: "Mid Air Swing."

Information sources:

1. Numerous posts by Chuck Westfall, Director/Technical Information Dept. Camera Division/Canon U.S.A., Inc. in the Canon forums of Rob Galbraith's website: http://www.robgalbraith.com
   
   
2. Photonotes article on AI Servo by Chuck Westfall