A cam mechanism for a lens barrel includes a cam ring including a cam groove which is open at one end, and a support ring supporting an imaging component, the support ring having a cam follower engageable with the at least one cam groove and movable therein towards and away from the open end wherein the support ring is movable along an axis relative to the cam ring without rotation. The cam ring and the support ring each have respective guide surfaces engageable with each other when the cam follower is moved in the region of the open end and shaped to guide the cam follower at the open end of the cam groove during engagement.