An initial focusing mechanism for a zoom lens system adapted to achieve the initial focusing by moving a variator lens and a compensator lens along an optical axis with respect to a focusing lens. A cylinder cam is slidably movable within a barrel along the optical axis. The cylinder cam carries the variator lens and the compensator lens and is rotatable with respect to the barrel during zooming so that the variator lens and compensator lens can be moved along the optical axis maintaining a given inter-lens optical relationship. A slide element is provided for moving the cylinder cam along the optical axis and locking the cylinder cam at a selected position relative to the barrel. For initial focusing, the cylinder cam is slidably moved along the optical axis which adjustably moves the variator lens and the compensator lens relative to the focusing lens. Upon completion of this adjustment, the cylinder cam is fixed to the barrel.