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If you look at the EF - EOS R mount adapter, you will find it has approximately ten vertical ring-shaped surfaces inside it (or actually 20, if you count both surfaces of a ring).

The simplest way to manufacture such a mount adapter would be to have just one cylindrical inner surface inside the adapter. However, Canon has opted to make the adapter more complex and have the approximately ten vertical ring-shaped surfaces inside it.

What is the purpose of these ring-shaped vertical surfaces? Are there any other mount adapters that have similar vertical surfaces?

Is it to keep dust out of the unprotected sensor by offering dust many alternative places to stick to, in case someone carries the mount adapter in the camera bag without caps on both ends?

Or is it to prevent light from reflecting from the walls of the adapter?

  • Many lens hoods have those inside them, too, to reduce reflections. – vlumi Jul 2 '19 at 12:44
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Is it to keep dust out of the unprotected sensor by offering dust many alternative places to stick to, in case someone carries the mount adapter in the camera bag without caps on both ends?

Or is it to prevent light from reflecting from the walls of the adapter?

Yes.

Mostly the latter, but it would also help with the former.

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