Bright and compact optical-compensation type zoom lens system

A bright and compact optical-compensation type zoom lens system comprising a first lens group I having positive refractive power, a second lens group II having negative refractive power, a third lens group III having positive refractive power, and a fourth lens group IV having positive refractive power, said zoom lens system being arranged to carry out zooming by keeping said second lens group II and said fourth lens group IV fixed and integrally moving said first lens group I and said third lens group III, said first lens group I comprising a cemented doublet consisting of a positive lens element and negative lens element, said second lens group II comprising at least one positive lens element and at least one negative lens element and comprising a cemented doublet having the surface on the object side thereof concave toward the object side, said third lens group III comprising a cemented doublet consisting of a positive lens element and negative lens element, said fourth lens group IV comprising at least one positive lens component and at least one negative lens component, said zoom lens system satisfying the following conditions (1) through (6):

• 0.5F.sub.W <.vertline.F.sub.II .vertline.<1.0F.sub.W ( 1)
• 0.6F.sub.W <F.sub.III <1.5F.sub.W ( 2)
• 0.6F.sub.W <r.sub.I-1 <2.0F.sub.W ( 3)
• 0.4F.sub.W <.vertline.r.sub.II .vertline.<1.2F.sub.W ( 4)
• .DELTA.N.sub.II +.DELTA.N.sub.III .ltoreq.0.4 (5)
• .DELTA..nu..sub.I >15 (6)
• wherein reference symbol F.sub.W represents the shortest focal length of the lens system as a whole, reference symbol F.sub.II represents the focal length of the second lens group II, reference symbol F.sub.III represents the focal length of the third lens group III, reference symbol r.sub.I-1 represents the radius of curvature of the surface on the object side of the first lens group I, reference symbol r.sub.II represents the radius of curvature of the surface on the object side of the cemented doublet constituting the second lens group II, reference symbol .DELTA.N.sub.II represents the difference between the mean value N'.sub.II of refractive indices of positive lenses constituting the second lens group II and mean value N".sub.II of refractive indices of negative lenses constituting the second lens group II, reference symbol .DELTA.N.sub.III represents the difference between the mean value N".sub.III of refractive indices of negative lenses constituting the third lens group III and mean value N'.sub.III of refractive indices of positive lenses constituting the third lens group III, and reference symbol .DELTA..nu..sub.I represents the value obtained by subtracting the Abbe's number .nu.".sub.I of the negative lens constituting the first lens group I from the Abbe's number .nu.'.sub.I of the positive lens constituting the first lens group I.