March 28, 2009

Twitter Q&A #3

Copyright 2004 All Rights Reserved - Scott Bourne

Copyright 2004 All Rights Reserved - Scott Bourne

This week I only have time for one Twitter question because it’s a complicated one.

Question #1 from @komantam – “Can you talk about how to best use AI Servo photography? ”

AI Servo AF is an autofocus system designed exclusively for Canon EOS cameras. It uses a form of Artificial Intelligence (AI) to determine the speed and direction of moving subjects, then focuses the camera lens to a predicted position (Focus Prediction Function) in order to increase the probability of obtaining a sharp photograph.

Similar systems are found on other camera brands but since you asked about AI Servo, I’ll limit my discussion to Canon.

First, let’s talk about when and when not to use it. If you’re photographing landscapes, portraits, still-lifes, product or catalog shots or any stationary object, then there’s no need for AI-Servo. Switch to Canon’s One-Shot mode.

AI Servo can be thought of as focus tracking. It is used to track a subject as it moves around the frame, or toward and away from the camera. It’s important to remember that this system uses algorithms that constantly PREDICT where a subject is about to be. It doesn’t guarantee sharp focus, it just predicts where the subject will be and focuses for that point. Depending on the type, speed and direction of movement, the system will have more or less success.

The two facets of AI Servo to think about are the predictive autofocusing and shutter release control.

According to Canon, predictive autofocusing happens when there’s a readable subject with predictable movement. This is the part of AI Servo that most people who’ve read even a few pages of their manual understand. The second part of the system, shutter release control, is less understood. In AI Servo, the CAMERA actually decides the shutter release timing (in a burst after the photographer makes the first shutter press.)

The net effect of systems like AI Servo is simple. Before this technology existed, you would pre-focus on an area where you thought the action would take place. Now you simply follow the action with the expectation that the auto-focus will keep up.

Now that I’ve covered all that – here are some things to remember that might actually help you take advantage of this system.

a. Know its limitations. The closer the action is to you, particularly if the action is moving TOWARD you, the less effective this system will be. AI Servo is better at side-to-side tracking than it is tracking objects moving directly toward or away from you.

b. To get the best results, hold the shutter button half way down when tracking a moving subject to acquire focus. This part is important – follow the subject for a second or two with the shutter pressed half-way down before actually firing the camera. This gives the AI time to build its predictions.

c. Consider modifying your custom functions to improve performance. Using C.Fn. 17 you can expand the AF activation area. The default is -0- and only the selected AF point or points are active. Setting this to -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.

This is just a basic primer on AI Servo. I hope you found it helpful. For more information consult the myriad of Canon technical white papers available online at the Canon USA site.

If you have questions you’d like me to answer, simply go to Twitter ( and make sure you follow me ( – then post your question and end it with the hash-tag #TWIPQA.


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Join the conversation! 4 Comments

  1. Wow, tip ‘c’ was a lot to digest! I’m just not that skilled with my camera yet.

    I was just reading through my manual out the AI Servo last night. This post was very helpful — thanks for writing it up!

  2. Unfortunately, C. fn. 17 on my 20D controls the behavior of the lens AF stop button. According to my manual, this button is only present on super telephoto lenses. In which model did Canon switch over to the behavior described above? I have had only limited success using AI autofocus on my 20D. I have found that I get better results using the zone focusing method described above.

  3. The system on my Nikon D300 is absolutely amazing at this in specific circumstances. Scott’s a,b, and c points are right on.

    Knowing how to use it can be very helpful. It’s also important because it helps you understand how the camera thinks when focussing. There’s a huge difference between shooting a bird with foreground/background branches and shoot something like a bicycler while panning. Knowing how to set your camera can make the difference. This knowledge can often be obtained through the manual the came with the camera.

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