Canon ULTRASONIC EF400mm f/2.8L IS III USM Instructions Manual page 10

4. Setting the Focusing Distance Range
Focusing distance range selector switch
You can set the focusing distance range with a
switch. By setting a suitable focusing distance
range, the actual autofocusing time will be
shorter.
Focusing distance range
① FULL (2.5 m – ∞)
② 2.5 m – 7 m
③ 7 m – ∞
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