BRASS T-RING FOR Nikon CAMERA allows you to attach your Canon EOS to your telescope via any T threaded camera adapter. Having the right equipment to shoot art-gallery images of the heavens starts with the right telescope, camera and camera adapter, and ends here, with the right T-mount for your make and model of camera. The bayonet is much wider than T2 so the adapter can be mounted completely into the camera body. The real optical length is 0mm.