This is a step-down ring from 55mm to 42mm. I bought it while struggling to connect a camera and a telescope. Now, I've given up on connecting them and decided to sell it. It sells for over 500 yen on Amazon, etc. Product Description by the Manufacturer: This product is a step-down ring with a 55mm male thread on the camera side and a 42mm female thread on the filter side. Therefore, if the camera (lens) you plan to use has a 55mm female thread and the filter you plan to use has a 42mm male thread, you can connect them using this step-down ring. [Note] When using a step-down ring with a lens, vignetting may occur depending on the lens, so please be careful. [Note] The thread pitch of this 42mm part is 0.75, which is different from the 1.0mm thread pitch used for M42 mounts, so please be careful. - Size: 55mm to 42mm - Type: Step-down ring Thank you for viewing.