Zoom lens

A zoom lens comprising, from an object side to an image plane side, a first lens unit of positive refractive power for focusing, a second lens unit of negative refractive power having an image magnification varying function, a third lens unit of negative refractive power for compensating for the image shift resulting from variation of the image magnification, a fourth lens unit of positive refractive power for making a light beam from said third lens unit to be an almost divergent light beam, and a fifth lens unit having an image forming function, said fifth lens unit including, from the object side of the image plane side, a first lens of which both lens surfaces are convex, a negative second lens having a concave surface facing the object side, a positive third lens which has a strong refracting surface facing the object side and of which both lens surfaces are convex, a negative fourth lens having a concave surface facing the image plane side, a fifth lens which has a strong refracting surface facing the image plane side and of which both lens surfaces are convex, and a positive sixth lens having a strong refractive surface directed toward the object side, and said zoom lens satisfying the following conditions:

• 1.05<f4/L<1.3
• 20.4<.vertline.R5,4/fw.vertline.<30.6
• 0<fw/R5,7<0.076
• 1.58<(N5,1+N5,3+N5,5+N5.6)/4<1.85
• where L is a distance from a convergent point of an on-axial light beam having passed through said first to third lens units to a front principal point of said fourth lens unit, f4 is a focal length of said fourth lens unit, N5,i is a refractive index of a medium of the i-th lens in said fifth lens unit, R5, i is a radius of curvature of the i-th lens surface in said fifth lens unit, and fw is the shortest focal length of the entire lens system.