Canon EOS R5 C User Manual page 64

2.
Attach the lens to the adapter.
Align the red or white mount index on the lens with the corresponding
mount index on the adapter and turn the lens as shown by the arrow
until it clicks into place.
(1) Red index
(2) White index
3.
Attach the adapter to the camera.
Align the red mount indexes (3) on the adapter and camera and turn
the lens as shown by the arrow until it clicks into place.
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