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Old Jan 18, 2010, 12:14 AM   #1
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Default AI Focus/Servo mode

I was playing around with the AI Focus and AI Servo modes on my camera today. My understanding is that AI Servo mode will track a moving object and keep it in focus once AF is locked. AI Focus mode determines whether AI Servo or OneShot focus is more appropriate.

My question is what is the proper procedure for using AI Focus/Servo modes? Do I AF an object or person, then pan with it while it moves? I've had it work a few times where I can hear the lens focusing, and the camera faintly beeps. Other times, I dont hear anything, so I wonder if I'm doing something incorrectly.
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Old Jan 18, 2010, 12:16 AM   #2
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Here is a link to a very good explanation of the different af modes.

http://canont1i.blogspot.com/2009/12...-use-with.html
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Old Jan 18, 2010, 12:31 AM   #3
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thanks. Glad to see I was on the right track. I find myself using AI Focus AF more often as I photograph people or objects that move unannounced, such as my children. Very handy. To further the question, if I use One Shot AF, and am looking to recompose a shot, how does the focus point work? IOW, if i focus on a subject using the center AF point, then recompose to move the subject to the right 1/3 of the frame, does the focus stay in the center point, or move to where the subject is now?
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Old Jan 18, 2010, 12:39 AM   #4
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It keeps that af lock on the last focus point as long as you have the shutter button press half way. I you release it will re focus on new points.
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Old Jan 18, 2010, 1:04 AM   #5
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wouldnt that risk the image being out of focus if i recompose the frame and the subject distance changes slightly?
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Old Jan 18, 2010, 1:08 AM   #6
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Not really if it is for a pose portrait or stationary object like landscape, seascapes, and building. A couple inches here or there will not throw off the focus.
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Old Jan 18, 2010, 2:50 AM   #7
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Focus-recompose definitely introduces focus errors.

http://visual-vacations.com/Photogra...pose_sucks.htm

However the effect gets worse with longer focal lengths and wide apertures, so with your camera and current lenses you probably won't notice it very much.

You will have to use focus-recompose to some extent. In general you should use the focus point closest to your intended point of focus. I seldom use more than one focus point, however the point that I use changes depending on where I want the point of focus to be.

Of course you have to balance that against the fact that your centre AF point is the most accurate. These things differ from camera to camera, so you will have to learn how to get the best results from your new camera for the type of shooting that you do.
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Last edited by peripatetic; Jan 18, 2010 at 2:53 AM.
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Old Jan 18, 2010, 12:54 PM   #8
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thats kind of what I thought.
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