Patent ID: 8149517

Claim:
A zoom lens unit, comprising in order from an object side to an image side: a first lens group having a positive refracting power; a second lens group having a negative refracting power; a third lens group having a positive refracting power; and a fourth lens group having a positive refracting power, an aperture stop being disposed between the second lens group and the third lens group, and when changing magnification from a wide-angle end to a telephoto end, an interval between the first lens group and the second lens group increases, an interval between the second lens group and the third lens group decreases, and an interval between the third lens group and the fourth lens group increases, and the first lens group and the third lens group are moved such that the first lens group and the third lens group locate nearer to the object side at the telephoto end than at the wide-angle end, wherein the third lens group has a positive lens made of an optical glass material which satisfies the following formulae: 1.52<nd<1.62 (1) 65.0<νd<75.0 (2) 0.015 <Pg,F −(−0.001802 ×νd+ 0.6483)<0.050 (3) where, nd represents a refractive index of the optical glass material, ν d represents an Abbe number of the optical glass material, and Pg,F represents a partial dispersion ratio of the optical glass material, the partial dispersion ratio, Pg,F, being defined as follows: Pg,F =( ng−nF )/( nF−nC ) where, ng, nF and nC represent refractive indexes of the optical glass material for g line, F line and C line, respectively.