Canon EOS 250D Advanced User's Manual page 106

AI Servo AF for Moving Subjects
This AF operation is suited for moving subjects when the focusing
distance keeps changing. While you hold down the shutter button
halfway, the camera will keep focusing on the subject continuously.
The exposure is set at the moment the picture is taken.
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When the AF point selection (= 108) is automatic, the camera first
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uses the center AF point to focus. During autofocusing, if the subject
moves away from the center AF point, focus tracking continues as long
as the subject is covered by another AF point.
With AI Servo AF, the beeper will not sound even when focus is achieved.
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Also, the focus indicator <o> in the viewfinder will not light up.
AI Focus AF for Switching the AF Operation
Automatically
AI Focus AF switches the AF operation from [One-Shot AF] to [AI
Servo AF] automatically if a still subject starts moving.
After the subject is in focus in One-Shot AF, if the subject starts
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moving, the camera will detect the movement, change the AF operation
automatically to AI Servo AF, and start tracking the moving subject.
When focus is achieved with AI Focus AF with the Servo operation active, the
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beeper will continue beeping softly. However, the focus indicator <o> in the
viewfinder will not light up. Note that focus lock is not possible in this case.
AF-Assist Beam with the Built-in Flash
Under low-light conditions, when you press the shutter button halfway, the
built-in flash may fire a brief burst of flashes. This illuminates the subject to
help autofocusing.
AF-assist beam will not be emitted from the built-in flash in <8: 35C>
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modes.
The AF-assist beam will not be emitted with AI Servo AF operation.
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The built-in flash makes a sound when firing continuously. This is normal and
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not a malfunction.
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