The invention relates to a high-performance, large-aperture yet wide-angle zoom lens system that can be used with an electronic image pickup device in particular, a method for focusing the same. In particular, the invention is concerned with a high-performance, large-aperture yet wide-angle zoom lens system which has a zoom ratio of the order of 3 at a diagonal field angle of 75° at its wide-angle end, so that it can be used with a single-lens reflex camera using an electronic image pickup device with the number of pixels being of the order of 6,000,000. The zoom lens system comprises a first lens group G1 having negative refracting power, a second lens group G2 having positive refracting power, a third lens group G3 having negative refracting power, a fourth lens group G4 having positive refracting power and a fifth lens group G5 having positive refracting power. Upon movement of an object point, focusing is carried out with the fifth lens group G5.