Af Stop Button - Canon EF300mm F2.8L IS USM Instructions Manual

Switching the focusing range
Before switching the focusing range, turn the
focusing ring to select a focusing distance
somewhere within the desired range.
If you do not set the lens to a focusing distance
that is within the selected focusing range,
autofocusing may stop when it reaches the
range boundary. This is not a fault. If it happens,
press the shutter button halfway down again.
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4. AF Stop Button

During autofocus operation, you can press the
AF stop button to temporarily pause autofocus.
If the shutter button is still pressed halfway
when the AF stop button is released,
autofocusing will continue as before.
The AF stop function also works in AI SERVO AF
mode.