A focusing system for long focal length objectives, having an intermediate tube between the camera and the objective proper. For purposes of focusing, there is a displaceable part movable longitudinally through the intermediate tube, and movement of this part serves to move the lens elements axially for focusing. A receiver portion on the bottom of the intermediate tube receives interchangeably, in quickly attachable and detachable manner, either one of two different focusing mechanisms, the first having the shape of a gun stock with a trigger-like member serving to focus the lens quickly but not necessarily precisely when quick picture shooting of moving objects is desired, the second mechanism having means for accurate focusing and a tripod thread socket. Provision is made for turning the camera 90 degrees to change the format of the picture. Mechanism is provided for pre-setting the diaphragm aperture, and for operating the diaphragm of the lens at the front end of the intermediate tube from the camera located at the rear end of the tube, with provision for adjusting these mechanisms properly when the camera is turned to change the format, without turning the objective at the front of the tube. It is not necessary to detach either the camera or the lens when changing from one focusing mechanism to the other, or when turning the camera to change the format.