Canon Yj18x9b4 - Manual

The ring closest to the front element. It rotates a full 360 degrees but has hard stops at the M.O.D. (0.9m) and Infinity.

: Extremely portable at approximately 1.2kg (2.65 lbs), making it ideal for shoulder-mounted shooting. Canon Yj18x9b4 Manual

The most common reason people search for the is that their lens "won't stay in focus" while zooming. This is a back focus (flange focal length) issue. The ring closest to the front element