Vello Tripod Collar A (White) for Canon 200mm f/2.8, 70-200mm f/4 & 400mm f/5.6

Improves Lens Balance
The ring secures your lens to the camera's tripod

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 adjust the balance.

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

Thumbscrews Tighten And Loosen Ring
Having thumbscrews control the ring makes it easy to adjust the tripod ring on the go