PURPOSE: To provide a bright zoom lens of which the aberration is satisfactorily corrected even if the F-number is as large as 2.8 over the entire zoom region by using a combined lens for the positive lens provided on the object side of the negative lens of a lens system of an Ernostar type and giving a large difference in the refractive index of the front and rear lenses of the combined lens so that said faces play the role of correcting a spherical aberration.

CONSTITUTION: The conditions expressed by the formulas ( I ) and (II) are both provided in order to correct the spherical aberration particularly at the telephoto end. If the correction of a spherical aberration is accomplished only by the face on the image side of the negative lens in the rear group, the power of said face is so strong that an astigmatism arises on the wide side and the bend of coma increases, thus leading to a considerable generation of flares. A large difference in refractive index is therefore given to both lenses of the combined lens disposed in the rear group so that the effect of correcting the spherical aberration is provided to said face as well. The condition of the formula (III) is to determine the paraxial basic constitution of the lens system. The condition of the formula (V) determines the thickness of the lens after the stop, i.e., the distance from the face on the object side of the positive combined lens in the rear group to the final face of the lens system. The condition of the formula (IV) regulates the combined lens so as to have a positive power as a whole.