This aluminum tripod collar mount ring is specially designed for Canon EF 70-300mm f/4-5.6L IS USM, balancing the lens & the camera, making it more convenient for you to adjust the lens after fixing on tripod. Ring inner diameter: 81.5mm, Ring width: 18mm, Ring thickness: 4.5 ~ 8mm, Color: White. Just spin the knob & pull it out, you can release the ring.