Image forming optical system and electronic image pickup apparatus using the same

1. An image forming optical system essentially consisting of, in order from an object side:

a first lens group having a positive refractive power;

a second lens group having a negative refractive power;

a third lens group having a positive refractive power; and

a fourth lens group having a positive refractive power, wherein

a refractive index for a d-line of a positive single lens having the largest refractive power among all lenses in the second lens group, is the smallest refractive index among all lenses in the second lens group,

the positive single lens having the largest refractive power among all lenses in the second lens group satisfies the following conditional expressions:

1.45 ≦nd p≦ 1.75  (1)

0.40 ≦fp /( fw·ft ) 1/2 ≦0.90  (4)

where,

nd p denotes the refractive index for the d-line of the positive single lens in the second lens group,

fp denotes a focal length of the positive single lens in the second lens group,

fw denotes a focal length of the overall optical system at a wide angle end, and

ft denotes a focal length of the overall optical system at a telephoto end.

2. The image forming optical system according to claim 1 , wherein the second lens group includes at least one concave lens having a smaller thickness at a center than a thickness at a center of the positive single lens.

3. The image forming optical system according to claim 1 , wherein the positive single lens having the largest refractive power among all lenses in the second lens group is positioned nearest to an image-plane side, in the second lens group.

4. The image forming optical system according to claim 1 , wherein the positive single lens having the largest refractive power among all lenses in the second lens group satisfies the following conditional expression:

10 ≦v dp≦ 35  (2)

where,

v dp denotes Abbe's number (n dp−1)/(n Fp−n Cp) for the d-line of the positive single lens in the second lens group, and

n Cp, and n Fp denote refractive indices for a C-line, and an F-line, respectively, of the positive single lens.

5. The image forming optical system according to claim 1 , wherein the positive single lens having the largest refractive power among all lenses in the second lens group satisfies the following conditional expression:

0.6 ≦θ gF≦ 0.75  (3)

where,

θ gF denotes a partial dispersion ratio of the positive single lens in the second lens group, and θ gF=(ng−nF)/(nF−nC),

where,

ng denotes a refractive index for a g-line of the positive single lens, nF denotes a refractive index for an F-line of the positive single lens, and nC denotes a refractive index for a C-line of the positive single lens.

6. An electronic image pickup apparatus comprising:

the image forming optical system according to claim 1 ; and

a photoelectric conversion element, wherein

the image forming optical system forms an image on the photoelectric conversion element.