Canon RF-S3.9 mm F3.5 STM DUAL FISHEYE Instructions Manual page 14

Checking/adjusting left and right focus
If the right lens focus goes notably out of range while correcting for focus differences in the left and right
z
lenses, slide the left-right focus difference adjustment mode switch back and forth three times within 6
seconds, starting from the ADJUST side.
This will reset the right lens position to its initial setting. After that, check and adjust the left and right focus
again.
When the left-right focus difference adjustment mode switch is set to the ADJUST side, the AF mode cannot
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be used. In addition, you cannot shoot still images and movies.
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