Microscope objective lens

1. A microscope objective lens comprising, in order from an object side,

a first lens group having positive refractive power,

a second lens group having positive refractive power, and

a third lens group having negative refractive power, wherein

the first lens group includes, on the most object side, a positive meniscus lens whose concave surface is directed to the object side,

the second lens group includes a diffractive optical element having positive refractive power, and

the diffractive optical element is arranged at a position closer to the image than a portion at which the diameter of a light flux passing through the first lens group and the second lens group is the largest,

the microscope objective lens satisfying the following expressions:

n 1≧1.8

0.5≦(− r 1)/ F≦ 4.2

ΦDOE/Φmax<0.9

where a refractive index relative to a d-line of a medium of the positive meniscus lens included in the first lens group is set as n1, a radius of curvature of the most object side lens surface of the first lens group is set as r1, a focal length of the whole lens system is set as F, the maximum diameter of the light flux passing through the first lens group and the second lens group is set as Φmax, and the maximum diameter of a light flux passing through a diffractive optical surface of the diffractive optical element is set as ΦDOE.

2. The microscope objective lens according to claim 1 , satisfying the following expression:

0.3≦ f 12/(− f 3)≦2

where a combined focal length of the first lens group and the second lens group is set as f12 and a focal length of the third lens group is set as f3.

3. The microscope objective lens according to claim 1 , satisfying the following expression:

100 ≦f DOE/ F≦ 1000

where a focal length of the diffractive optical element is set as fDOE.

4. The microscope objective lens according to claim 1 , wherein

the third lens group includes at least one cemented lens comprising a positive lens and a negative lens,

the microscope objective lens satisfying the following expression:

−0.0035≦Δθ gF 3/Δν3≦−0.002

where a partial dispersion ratio θgF is defined as θgF=(ng−nF)/(nF−nC), with a refractive index relative to a g-line being set as ng, a refractive index relative to an F-line being set as nF, and a refractive index relative to a C-line being set as nC; a difference in the partial dispersion ratio between the positive lens and the negative lens which configure the cemented lens provided in the third lens group is set as ΔθgF3; and a difference in the Abbe number between the positive lens and the negative lens is set as Δν3.

5. The microscope objective lens according to claim 1 , wherein

the second lens group includes at least one cemented lens comprising a positive lens and a negative lens,

the microscope objective lens satisfying the following expression:

−0.0025≦Δθ gF 2/Δν2≦−0.0006

where the partial dispersion ratio θgF is defined as θgF=(ng−nF)/(nF−nC), with the refractive index relative to the g-line being set as ng, the refractive index relative to the F-line being set as nF, and the refractive index relative to the C-line being set as nC; the difference in the partial dispersion ratio between the positive lens and the negative lens which configure the cemented lens provided in the second lens group is set as ΔθgF2, and the difference in the Abbe number between the positive lens and the negative lens is set as Δν2.

6. The microscope objective lens according to claim 1 , satisfying the following conditional expressions:

2≦ f 1/ F≦ 10

15≦ f 2/ F≦ 30

where a focal length of the first lens group is set as f1 and a focal length of the second lens group is set as f2.