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Canon EOS 40D Guide to Digital Photography by David D. Busch

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AI Servo AF

This mode, also known as Continuous Autofocus is the mode to use for sports and other fast-moving subjects. In this mode, once the shutter release is partially depressed, the camera sets the focus but continues to monitor the subject, so that if it moves or you move, the lens will be refocused to suit. Focus and exposure aren’t really locked until you press the shutter release down all the way to take the picture. You’ll often see Continuous Autofocus referred to as release priority. If you press the shutter release down all the way while the system is refining focus, the camera will go ahead and take a picture, even if the image is slightly out of focus. You’ll find that AI Servo AF produces the least amount of shutter lag of any ...

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