The Tripod Mount Ring C from Canon is designed for the EF 70-300mm f/4-5.6L IS USM and EF 28-300mm f/3.5-5.6L IS USM lenses. The ring makes rotating your lens easier and the built-in tripod collar improves lens balance by screwing into the tripod mount.
Improves Lens Balance
The ring secures your lens to the camera's tripod
Thumbscrews Tighten and Loosen Ring
Having thumbscrews control the ring make it easy to adjust the tripod ring on the go
Gives Your Camera Setup Better Balance
The ring adds stability to your camera and lens. Due to its weight, when the lens is mounted with a camera on a tripod, the center of gravity is shifted to the lens. The ring attaches to the tripod and can be shifted to any position on the lens, ensuring the weight is evenly distributed
Rotation
Having the camera secured with a ring around the lens makes it possible to rotate the lens between landscape and portrait modes without removing the camera from the tripod. The friction thumbscrews can then be tightened to secure the camera in the desired position. When not needed, the ring can be rotated to an out of the way position
Hinged Design
The hinge provides the ability to keep the lens attached to the camera while disconnecting the lens from the tripod