An optical device includes an optical system, a diaphragm unit, a diaphragm unit, a focus-lens driving unit, a diaphragm driving unit, and a diaphragm control unit. The diaphragm control unit controls the diaphragm driving unit in accordance with the position to which the focus lens group has been moved by the focus-lens driving unit, thereby controlling the aperture opening of the diaphragm unit and ultimately limit a light beam passing through the camera system.