High variable-power ratio, high aperture ratio, zoom optical system

A high variable-power ratio, high aperture ratio, zoom optical system for a lens shutter camera, comprising a first zoom lens group, a second zoom lens group, a third zoom lens group and a fourth zoom lens group arranged on an optical axis in that order from an object side;

• wherein:
• said first zoom lens group comprises a negative lens and a positive lens arranged in that order from said object side, said first zoom lens group acting as a zoom lens group having a positive focal length;
• said second zoom lens group comprises a negative lens and a positive lens arranged from said object side in that order, said second zoom lens group acting as a zoom lens group having a negative focal length;
• said third zoom lens group comprises a double concave lens and a double convex lens arranged in that order from said object side, said third zoom lens group acting as a zoom lens group having a positive focal length, said third zoom lens group being provided with a diaphragm;
• said fourth zoom lens group comprises a positive lens and a negative lens arranged from said object side in that order, said fourth zoom lens group acting as a zoom lens group having a negative focal length;
• wherein, when zooming is performed from a wide-angle-end formation to a telephoto-end formation, at least said first zoom lens group and said fourth zoom lens group move toward said object side, also, with movement of both zoom lens groups, an axial distance between said first zoom lens group and said second zoom lens group increases, and also, an axial distance between said third zoom lens group and said fourth zoom lens group decreases;
• wherein:
• each of said third zoom lens group and said fourth zoom lens group includes at least one aspherical surface,
• wherein the following conditions are satisfied:
• 0.2<.vertline.f.sub.1 /f.sub.2 .vertline.<1.6;
• (1)
• 0.15<f.sub.3 /f.sub.T <0.35;
• (2)
• 0.2<.vertline.f.sub.4 /f.sub.T .vertline.<0.4;
• and (3)
• 1<.vertline.R.sub.a /R.sub.b .vertline.<6;
• (4)
• where:
• f.sub.1 represents a focal length of said first zoom lens group;
• f.sub.2 represents a focal length of said second zoom lens group;
• f.sub.3 represents a focal length of said third zoom lens group;
• f.sub.4 represents a focal length of said fourth zoom lens group;
• f.sub.T represents a focal length of the optical system at said telephoto-end formation;
• R.sub.a represents one of a radius of curvature and a radius of paraxial curvature of the object side lens face a of a fifth lens counted from said object side;
• and
• R.sub.b represents one of a radius of curvature and a radius of paraxial curvature of an image side lens face b of a sixth lens counted from said object side.