Abstract: A relay optical system includes, in order from, an object side, a first cemented lens, a second cemented lens having a positive refractive power, a third cemented lens which is plane-symmetric to the second cemented lens, and a fourth cemented lens which is plane-symmetric to the first cemented lens, the first cemented lens includes a first lens having a positive refractive power and a second lens having a negative refractive power, the second cemented lens includes a third lens and a fourth lens, a shape of the first lens is a biconvex shape, a shape of the second lens is a biconcave shape, and the following conditional expression (1), in which f2 denotes a focal length of the second lens and fCL12 denotes a combined focal length of the first cemented lens and the second cemented lens, is satisfied: ?0.4<f2/fCL12<?0.1??(1).

Abstract: The relay optical system includes in order from an object side, a first cemented lens, a second cemented lens having a positive refractive power, a third cemented lens which is plane-symmetric to the second cemented lens, and a fourth cemented lens which is plane-symmetric to the first cemented lens, and the first cemented lens includes a first lens having a positive refractive power and a second lens having a negative refractive power, and the second cemented lens includes a third lens and a fourth lens, and a shape of the first lens is a biconvex shape, and a shape of the second lens is a biconcave shape, and the following conditional expression (1) is satisfied: ?0.4<f2/fCL12<?0.1 ??(1).

Abstract: An objective optical system for endoscope includes, in order from an object side, a front lens group having a negative refractive power as a whole, an aperture stop, and a rear lens group having a positive refractive power as a whole. The front lens group includes, in order from the object side, a single first lens having a negative refractive power and a single second lens having a positive refractive power. The rear lens group includes a single third lens having a positive refractive power, and a cemented lens of a fourth lens having a positive refractive power and a fifth lens having a negative refractive power. An object-side surface of the first lens is a flat surface, the second lens has a meniscus shape having a convex surface directed toward an image side, and the third lens has a biconvex shape.

Abstract: There is provided a small-sized and bright objective optical system for endoscope with a wide angle of view and thin diameter, which enables to achieve a highly-defined image.

Abstract: An object of the present invention is to reduce the diameter, to make focus adjustment easy, and to achieve bright, wide-angle, high-resolution image quality. Provided is an endoscope objective optical system including, in this order from the object side, a front group having negative refractive power, an aperture stop, and a positive rear group. The front group includes, in this order from the object side, a first lens, which is a single lens having negative refractive power, and a second lens, which is a single lens having positive refractive power. The rear group includes a third lens, which is a single lens having positive refractive power, a cemented lens formed of a fourth lens having positive refractive power and a fifth lens having negative refractive power, and a sixth lens having positive refractive power.

Abstract: An object of the present invention is to reduce the diameter, to make focus adjustment easy, and to achieve bright, wide-angle, high-resolution image quality. Provided is an endoscope objective optical system including, in this order from the object side, a front group having negative refractive power, an aperture stop, and a positive rear group. The front group includes, in this order from the object side, a first lens, which is a single lens having negative refractive power, and a second lens, which is a single lens having positive refractive power. The rear group includes a third lens, which is a single lens having positive refractive power, a cemented lens formed of a fourth lens having positive refractive power and a fifth lens having negative refractive power, and a sixth lens having positive refractive power.

Abstract: Provided is 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, where f is a focal length of an entire system, f4 is a focal length of the fourth group, and f2 is a focal length of the second group. ?0.5<f/f4<?0.001??(7) 0.1?|f4/f2|?5.

Abstract: Provided is 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, where f is a focal length of an entire system, f4 is a focal length of the fourth group, and f2 is a focal length of the second group. ?0.5<f/f4<?0.001??(7) 0.