Microscope objective lens

1. A microscope objective lens, wherein the microscope objective lens satisfies the following conditional expressions (1) and (2)

0.05<NA<0.4  (1)

3 mm< D /NA<50 mm  (2)

where,

NA denotes a numerical aperture on an object side of the microscope objective lens, and

D denotes a sum of thicknesses of all lenses included in the microscope objective lens, and wherein a parfocalizing distance PD of the microscope objective lens is calculated by the following expression (5)

PD=45+15 m  (5)

where,

m is −1, 0, 1, 2, 3, or 4.

2. The microscope objective lens according to claim 1 , wherein the microscope objective lens satisfies the following conditional expression (3)

0.5< D/AD< 7  (3)

where,

D denotes the sum of thicknesses of all lenses included in the microscope objective lens, and

AD denotes a sum of air spaces between lenses of the microscope objective lens.

3. The microscope objective lens according to claim 1 , wherein the microscope objective lens satisfies the following conditional expression (4)

0.2< D/f< 2  (4)

where,

D denotes the sum of thicknesses of all lenses included in the microscope objective lens, and

F denotes a focal length of the overall microscope objective lens system.

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

a lens which forms the microscope objective lens is made of at least two types of glass, and

a refractive index of the lens for a d-line is not more than 1.7.

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

the microscope objective lens is accommodated in a microscope objective lens unit, and

the microscope objective lens unit has a detaching portion which is detachable from an observation apparatus.