Canon EF 100-400mm f/4.5-5.6L IS USM Instruction page 6

3. Switching the Focusing
Distance Range
You can set the focusing distance range to 1.8
m/5.9 ft. to infinity or 6.5 m/21.3 ft. to infinity. By
setting the suitable focusing distance range, the
actual autofocusing time will be shorter.
If you use AF from outside the specified focusing
distance range, the lens may stop focusing when it
reaches the limit of the range. This is not a
malfunction. If this occurs, press the shutter button
halfway down again.
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