A zoom lens system having a range in the order of 1:9 with a relative aperture of 1:1.4, including a variable afocal front attachment and a basic objective which is designed to improve the Petzval sum with front and rear groups each consisting of three lens elements of positive, positive and negative powers in this order from the front by fulfilling the following relationships with its individual focal length Fr: ##EQU1## wherein Ff is the focal length of the front group; r4 is the radius of curvature of the rear surface of the second positive lens in the front group; r5 is the radius of curvature of the front surface of the negative lens in the front group; r8 and r10 are the radii of curvature of the rear surfaces of the first and second positive lenses in the rear group respectively; and r11 is the radius of curvature of the front surface of the negative lens in the rear group.