The Canon Tripod Mount Ring A II (W)'s main purpose is to provide a better balance when mounted on a tripod. The 70-200 f/4 lenses do not require this ring to be mounted on a tripod - the camera body tripod insert can be used. But, this setup is front heavy. A top-notch ball head can handle this load, but a lesser one will drop some amount after it is tightened. Care must also be taken to not tip the off-balance tripod forward. By moving the tripod mount to a centrally-positioned tripod ring (collar), balance is restored.
The advantages do not stop here. Since the tripod ring freely rotates, a nice amount of setup flexibility is gained. With the Canon Tripod Mount Ring A II (W) in use, switching from horizontal to vertical orientation is fast and easy. Otherwise, a major tripod head adjustment is required (such as dropping a ball head into the drop-notch).
The thumbscrew tightens/loosens the tripod ring friction as needed. Simply rotate the ring to the desired position and lock it tight. If not needed, rotate the ring to an out-of-the-way position when not in use. Alternatively, loosen the tripod ring and pull out on the thumbscrew to completely release the ring from the lens. This release is accomplished via a hinge (noticeable in the pics). A hinged design allows the Canon Tripod Mount Ring A II (W) to be removed without the need to unmount the lens from the camera.
A tripod ring allow various accessories to be attached to the lens - flash brackets being one of the most popular. I use a Wimberley P20 Lens Plate on my Canon Tripod Mount Ring A II (W). This allows me to quickly mount the lens to any common Arca-Swiss-compatible quick-release clamp (Arca-Swiss, Wimberley, RRS ...) on a ball head, monopod clamp, flash bracket ...
At one point, Canon had two Tripod Mount Ring A (W) models and one (B) model (W = White, B = Black). Both (W) models are shown in the pictures below (II is on the right), are interchangeable and are basically the same in their functionality. The II features a more-modern styling and a smaller thumbscrew (slightly more force required to lock it down).
The Canon Tripod Mount Ring A II (W) does not deliver the ultimate in tripod ring smoothness, but it works very well. Build quality is very nice. The features it delivers are worth the price to me.
Bringing you this site is my full-time job (typically 60-80 hours per week). Thus, I depend solely on the commissions received from you using the links on this site to make any purchase. I am grateful for your support! - Bryan
Where you buy your gear matters. You expect to get what you ordered and you want to pay a low price for it. The retailers I recommend below are the ones I trust for my purchases. Get your Canon Tripod Mount Ring A II (W) now from:B&H Photo