Patent ID: 8520135

Claim:
An image forming optical system comprising: a lens group I located closest to its object side; an aperture stop; a lens group A disposed between the lens group I and the aperture stop and having a negative refracting power as a whole, wherein the lens group A includes a cemented lens component made up of a positive lens LA and a negative lens LB that are cemented together, the distance between the lens group I and the lens group A on the optical axis changes for zooming, the cemented lens component has an aspheric cemented surface, and when the shape of the aspheric surface is expressed by the following equation (1) with a coordinate axis z taken along the optical axis and a coordinate axis h taken along a direction perpendicular to the optical axis: z=h 2 /R[ 1+{1−(1+ k ) h 2 /R 2 } 1/2 ]+A 4 h 4 +A 6 h 6 +A 8 h 8 +A 10 h 10 + (1), where R is the radius of curvature of a spherical component on the optical axis, k is a conic constant, A 4 , A 6 , A 8 , A 10 , . . . are aspheric coefficients, and a deviation is expressed by the following equation (2): Δ z=z−h 2 /R[ 1+{1− h 2 /R 2 } 1/2 ] (2), the following conditional expression (3a) or (3b) is satisfied: when R c ≧0, Δ z C ( h )≦(Δ z A ( h )+Δ z B ( h ))/2 (where h= 2.5 a ) (3a), when R c ≦0, Δ z C ( h )≧(Δ z A ( h )+Δ z B ( h ))/2 (where h= 2.5 a ) (3b), where z A expresses the shape of the air-contact surface of the positive lens LA according to equation (1), z B expresses the shape of the air-contact surface of the negative lens LB according to equation (1), z C expresses the shape of the cemented surface according to equation (1), Δz A is the deviation of the air-contact surface of the positive lens LA according to equation (2), Δz B is the deviation of the air-contact surface of the negative lens LB according to equation (2), Δz C is the deviation of the cemented surface according to equation (2), R c is the paraxial radius of curvature of the cemented surface, and a is a value expressed by the following equation (4): a =( y 10 ) 2 ·log 10 γ/fw (4), where y 10 is the largest image height, fw is the focal length of the entire image forming optical system at the wide angle end, and γ is the zoom ratio (i.e. {the focal length of the entire system at the telephoto end}/{the focal length of the entire system at the wide angle end}) of the image forming optical system, where z(0)=0 holds for all the surfaces because the point of origin is set at the vertex of each surface.