Image pickup apparatus

1. An image pickup apparatus including a variable power imaging optical system comprising 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 negative refractive power, wherein

the fourth lens group comprises two lens components namely, a positive meniscus lens having a convex surface directed toward an image side, and a biconcave negative lens in order from the object side, or, one lens component in which, the positive meniscus lens having the convex surface directed toward the image side, and the biconcave negative lens are cemented in order from the object side, and the fourth lens group satisfies the following conditional expressions (1) and (2)

N d4 ≦−0.015×ν d4 +2.04  (1)

−2.5≦ f G4 /L≦− 1.5  (2)

where,

N d4 denotes a refractive index of a glass material of the positive meniscus lens in the fourth lens group of the image forming optical system,

ν d4 denotes Abbe's number (n d1 −1)/(n F1 −n C1 ) for a d-line of the glass material of the positive meniscus lens in the fourth lens group of the image forming optical system,

f G4 denotes a focal length of the fourth lens group in the image forming optical system,

L denotes a length of a diagonal of an image pickup element, and

n d1 , n C1 , and n F1 denote refractive indices of the positive meniscus lens for the d-line, a C-line, and an F-line respectively.

2. The image pickup apparatus according to claim 1 , wherein the image forming optical system satisfies the following conditional expression (3)

0.66≦θ gF4p ≦0.75  (3)

where,

θ gF4p denotes a partial dispersion ratio (n g −n F )/(n F −n C ) of the positive meniscus lens in the fourth lens group of the image forming optical system,

where,

n g denotes a refractive index for a g-line, of the positive meniscus lens,

n F denotes a refractive index for the F-line, of the positive meniscus lens, and

n C denotes a refractive index for the C-line, of the positive meniscus lens.

3. The image pickup apparatus according to claim 1 , wherein the image forming optical system satisfies the following conditional expression (4)

−7.0≦( R b+ +R a+ )/( R b+ −R a+ )≦2.0  (4)

where,

R a+ denotes a radius of curvature on the object side, of the positive meniscus lens in the fourth lens group of the image forming optical system, and

R b+ denotes a radius of curvature on the image side, of the positive meniscus lens in the fourth lens group of the image forming optical system.

4. The image pickup apparatus according to claim 1 , wherein

the third lens group of the image forming optical system, in order from the object side, comprises an aperture stop, a front group of a lens component having a positive refractive power, and a rear group of a lens component having a positive refractive power, and comprises a lens component having a negative refractive power nearest to the image side of the front group.

5. The image pickup apparatus according to claim 1 , wherein

the third lens group of the image forming optical system, in order from the object side, comprises an aperture stop, a front group of a positive lens component having a positive refractive power, and a rear group of a lens component having a positive refractive power, and comprises a lens having a negative refractive power nearest to the object side of the rear lens group.

6. The image pickup apparatus according to claim 1 , wherein the second lens group of the image forming optical system, in order from the object side, comprises a lens component having a negative refractive power, a lens component having a positive refractive power, and a lens component having a negative refractive power.

7. The image pickup apparatus according to claim 1 , wherein the first lens group of the image forming optical system, in order from the object side, comprises two lens components namely, a lens component having a positive refractive power and a lens component having a negative refractive power or one lens component in which, the lens component having a positive refractive power and the lens component having a negative refractive power are cemented.