Imaging optical system

1. An imaging optical system, characterized by comprising, in order from an object side thereof, a first lens group having positive refracting power, a stop, a second lens group having positive refracting power and a third lens group having negative refracting power, wherein:

said first lens group comprises a cemented lens consisting of a first lens and a second lens;

said second lens group comprises a third lens;

said third lens group comprises a fourth lens, wherein said fourth lens has an object-side surface defined by a concave surface when viewed from the object side on and off an optical axis of the optical system, and an image-side surface defined by an aspheric surface that is convex when viewed from the object side on the optical axis and has one or more inflection points off the optical axis;

said cemented lens is made up by cementing together said first lens formed of a resin layer and said second lens formed of a resin layer;

said first lens is a positive lens;

said second lens satisfies the following condition (5); and

said cemented lens satisfies the following condition (6):

0.05≦t≦0.2  (5)

20 ≦v 2 −v 1≦40  (6)

where t is a thickness (in mm) of said second lens,

v 1 is an Abbe constant of said second lens, and

v 2 is an Abbe constant of said first lens.

2. The imaging optical system according to claim 1 , which satisfies the following conditions (1) and (2):

D 2 /D 1<4  (1)

0.1 <R 6 /R 7<1.5  (2)

where D 1 is a length, as calculated on an air basis, of an axial chief ray between an image-side surface of said third lens and an object-side surface of said fourth lens,

D 2 is a length, as calculated on an air basis, of an off-axis chief ray having a maximum image height between the image-side surface of said third lens and the object-side surface of said fourth lens,

R 6 is a radius of curvature of the image-side surface of said third lens, and

R 7 is a radius of curvature of the object-side surface of said fourth lens.

3. The imaging optical system according to claim 1 , characterized by satisfying the following condition (3):

0.5 <IH /exp<1.5  (3)

where exp is a distance from a position of an exit pupil having a maximum angle of view to an image plane, and

IH is an image height.

4. An imaging optical system, characterized by comprising, in order from an object side thereof, a first lens group having positive refracting power, a stop, a second lens group having positive refracting power and a third lens group having negative refracting power, wherein:

said first lens group comprises a cemented lens consisting of a first lens and a second lens;

said second lens group comprises a third lens;

said third lens group comprises a fourth lens, wherein said fourth lens has an object-side surface defined by a concave surface when viewed from the object side on and off an optical axis of the optical system, and an image-side surface defined by an aspheric surface that is convex when viewed from the object side on the optical axis and has one or more inflection points off the optical axis; and

the object-side surface of the fourth lens is made up of an aspheric surface, with satisfaction of the following condition (4):

−10 <A 4<0, −10 <A 6<0  (4)

where r is a paraxial radius of curvature of the aspheric surface,

K is a conical coefficient,

Ai (i=4, 6, 8, 10) is an i-th order aspheric coefficient,

h is a height from an optical axis, and

Z is a distance from a tangent plane of a point on the aspheric surface at the height h from the optical axis and an apex of the aspheric surface, and

wherein a surface shape of the object-side surface of said fourth lens is represented by the following aspheric equation:

Z =( h 2 /r )/[1+{1−( K+ 1)·( h/r ) 2 } 1/2 ]+A4 ×h 4 +A 6 ×h 6 +A 8 ×h 8 +A 10 ×h 10 .