Patent ID: 8446511

Claim:
An image forming optical system comprising in order from an object side: a first lens group G 1 having a positive refractive power; a second lens group G 2 having a negative refractive power; an aperture stop; a third lens group G 3 having a positive refractive power; and a fourth lens group G 4 having a negative refractive power, wherein at the time of zooming, air distances between the lens groups are variable, and an air lens nearest to an image side in the third lens group G 3 has a shape of a convex lens, and the fourth lens group G 4 comprises one lens component, and is movable even at the time of focusing, and satisfies the following conditional expressions 0.5<( R 4 F+R 4 R )/( R 4 F−R 4 R )<8.0 (1) −12.0<( R 4 LAF+R 4 LAR )/( R 4 LAF−R 4 LAR )<−2.0 (2) where, R 4 F denotes a radius of curvature on an optical axis of a surface nearest to an object side of the fourth lens group G 4 , R 4 R denotes a radius of curvature on an optical axis of a surface nearest to an image side of the fourth lens group G 4 , R 4 LAF denotes a radius of curvature on an optical axis of a surface on the object side of a positive lens LA which is cemented to the fourth lens group G 4 , and R 4 LAR denotes a radius of curvature on an optical axis of a surface on the image side of the positive lens LA which is cemented to the fourth lens group G 4 .