An imaging optical system includes three lens elements. The three lens elements, in order from an object side to an image side, are a first lens element having an object-side surface facing the object side and an image-side surface facing the image side, a second lens element having an object-side surface facing the object side and an image-side surface facing the image side, and a third lens element having an object-side surface facing the object side and an image-side surface facing the image side. The third lens element has positive refractive power, the imaging optical system has a total of three lens elements, and there are air gaps between paraxial regions of the first lens element, the second lens element and the third lens element.

An imaging optical system includes three lens elements. The three lens elements, in order from an object side to an image side, are a first lens element having an object-side surface facing the object side and an image-side surface facing the image side, a second lens element having an object-side surface facing the object side and an image-side surface facing the image side, and a third lens element having an object-side surface facing the object side and an image-side surface facing the image side. The third lens element has positive refractive power, the imaging optical system has a total of three lens elements, and there are air gaps between paraxial regions of the first lens element, the second lens element and the third lens element.

An ocular lens includes from an object side: a first lens group having negative refracting power, and a second lens group having positive refracting power. An object-side focal plane of the second lens group is positioned between the first lens group and the second lens group. The first lens group includes, from the object side: a meniscus first A lens component whose convex surface is directed toward the object side, and a first B lens component having negative refracting power. The second lens group includes, in a position closest to the object side, a meniscus second A lens component whose concave surface is directed toward the object side, and satisfies conditional expressions.

An ocular lens includes from an object side: a first lens group having negative refracting power, and a second lens group having positive refracting power. An object-side focal plane of the second lens group is positioned between the first lens group and the second lens group. The first lens group includes, from the object side: a meniscus first A lens component whose convex surface is directed toward the object side, and a first B lens component having negative refracting power. The second lens group includes, in a position closest to the object side, a meniscus second A lens component whose concave surface is directed toward the object side, and satisfies conditional expressions.

An objective optical system for endoscope includes a first lens group having a positive refractive power, a second lens group having a negative refractive power, and a third lens group having a positive refractive power. At a time of focusing, the first lens group and the third lens group are fixed, and the second lens group moves. The third lens group consists of a front group having a positive refractive power and a rear group having a positive refractive power. The front group includes one cemented lens and the rear group includes one single lens, and the following conditional expressions are satisfied:
1fG3f/fG3r5,
0.1dG3fr/dG3r1,
8fc/rc2, and
7fG2/fG32.

An objective optical system for endoscope includes a first lens group having a positive refractive power, a second lens group having a negative refractive power, and a third lens group having a positive refractive power. At a time of focusing, the first lens group and the third lens group are fixed, and the second lens group moves. The third lens group consists of a front group having a positive refractive power and a rear group having a positive refractive power. The front group includes one cemented lens and the rear group includes one single lens, and the following conditional expressions are satisfied:
1fG3f/fG3r5,
0.1dG3fr/dG3r1,
8fc/rc2, and
7fG2/fG32.

Binoculars and an ocular lens have an excellent optical performance. Binoculars include: a first lens-barrel and a second lens-barrel having an objective lens and an ocular lens; and an Abbe-Koenig prism which is an erecting optical element included in the first lens-barrel and the second lens-barrel, and is characterized in that the product of an effective diameter (mm) of the objective lens and the real field of view () is 450 mm or larger.

An objective optical system for endoscope includes, a first lens group having a positive refractive power, a second lens group having a negative refractive power, and a third lens group having a positive refractive power. At a time of focusing, the first lens group and the third lens group are fixed, and the second lens group moves. The third lens group consists of a front group having a positive refractive power and a rear group having a positive refractive power. The front group includes one cemented lens and the rear group includes one single lens, and the following conditional expressions (1-1), (1-2), (4-1), and (4-2) are satisfied:

1fG3f/fG3r5(1-1)

0.1dG3fr/dG3r1(1-2)

8fc/rc2(4-1)

7fG2/fG32(4-2).

An objective optical system for endoscope includes, a first lens group having a positive refractive power, a second lens group having a negative refractive power, and a third lens group having a positive refractive power. At a time of focusing, the first lens group and the third lens group are fixed, and the second lens group moves. The third lens group consists of a front group having a positive refractive power and a rear group having a positive refractive power. The front group includes one cemented lens and the rear group includes one single lens, and the following conditional expressions (1-1), (1-2), (4-1), and (4-2) are satisfied:

1fG3f/fG3r5(1-1)

0.1dG3fr/dG3r1(1-2)

8fc/rc2(4-1)

7fG2/fG32(4-2).