This brand new 55mm metal adapter ring is fully compatible with the Cokin system.
It was made to fit the Cokin P series filter holder
The first thing to do is to insert the adapter ring into the only slot in the back of the holder as shown below.
Then, push the adapter ring all the way down into place Once the adapter ring is in place,
you can mount the holder/ring combo to your camera.
The adapter ring thread allows you to use a minimum number of step-up rings.
Otherwise,the holder will be pushed away from the lens and vignetting is possible.