rection perpendicular to the optical axis, r is a radius of paraxial curvature, k is a conical coefficient, and a, b, c, and d are aspherical coefficients. [0128] First Embodiment [0129] As shown in FIGS. 2A-2C, a zoom optical system according to the first embodiment of the present invention is a twofold zoom optical system including, in order from the object side, a first group G1 having a negati