Canon RF 100-500mm F4.5-7.1 L IS USM Instructions Manual page 18

Extenders (Sold separately)
If the focusing distance is set at 300mm when the extender is detached, there are cases in which it will not
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be possible to turn the zoom ring toward the WIDE setting from the 300mm point after it has been removed.
In either of these events, turn the zoom ring slightly toward the TELE setting to release the zoom ring so
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that it can be turned to the WIDE setting. You may hear a noise during this, but it will have no effect on
operations.
The speed of the AF will be slower when an extender is in use on account of controllability.
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ENG-17