Objective optical system

1. An objective optical system comprising, in order from an object side:

an aperture stop;

a positive first group;

a second group; a positive third group; and

a fourth group,

wherein the first group is formed of a single meniscus lens or plano-convex lens, whose convex surface faces an image side,

the second group is formed of a single lens,

the third group is formed of a cemented lens consisting of a positive lens and a negative lens,

the fourth group is formed of a single lens, and

Conditional Expressions (7) and (8) below are satisfied:

−0.5 <f/f 4<−0.001, and  (7)

0.1 ≦|f 4 /f 2|≦5,

where f is a focal length of an entire system,

f 4 is a focal length of the fourth group, and

f 2 is a focal length of the second group.

2. The objective optical system according to claim 1 , wherein Conditional Expression (9) below is satisfied:

0.3 <f/f 3<0.8,  (9)

where f 3 is a focal length of the third group.

3. The objective optical system according to claim 2 , wherein Conditional Expression (10) below is satisfied:

0.15 <f/f 1<1.1,  (10)

where f 1 is a focal length of the first group.

4. The objective optical system according to claim 3 , wherein the first group satisfies Conditional Expression (11) below:

1≦|( C 1 +C 2)/( C 1 −C 2)|<40,  (11)

where C 1 is a curvature of an object-side surface of the first group, and

C 2 is a curvature of an image-side surface of the first group.