Abstract: A zoom lens system shifts the position of front and rear lens groups relative to each other in the direction of the optical axis to change its focal length, by moving a lens barrel back and forth in the axial direction. The lens barrel is formed with cutouts in its rear portion for receiving therein parts and elements projecting forwardly from the plane in which an exposure aperture is located when the lens barrel is retracted, so as to reduce the thickness of the camera with the retracted zoom lens. A focal length changing mechanism comprises a fixed cam and a lever attached to the lens barrel which is turned by the cam when the lens barrel moves back and forth to shift one lens group, thereby changing the focal length of the zoom lens to desired focal lengths. To correct the changes in focus of the zoom lens caused by the change of focal length, an adjusting member is provided in the focal length changing mechanism.