The meta bones speed booster that was built for the original black magic design pocket cinema camera. This works with the original Pocket camera, and the current Black magic pocket cinema camera 4K, and many others that don't have a mirror behind the lens. It won't work with cameras that have a built in mirror, as the rear element extends pretty deep inside the camera, this improves the optical quality over the ones that have the rear element further away, but limits the cameras you can use it on.
New or Used:
Used