Abstract: An endoscope includes: a light guide fiber having an exit end through which illumination light is emitted; a transparent distal-end element configured to hold the exit end of the light guide fiber and made of a transparent material that allows the illumination light emitted from the exit end to pass therethrough; and an objective optical system inserted through a through-hole provided in the transparent distal-end element, wherein at least a portion of the illumination light enters the objective optical system through a boundary between the transparent distal-end element and the objective optical system, and a refractive index of the transparent material constituting the transparent distal-end element is larger than a refractive index of at least one material in contact with an inner surface of the through-hole.

Abstract: An objective optical system includes, in order from an object side to an image side, a front group having a negative refractive power, an aperture stop, and a rear group having a positive refractive power. The front group includes a negative single lens, the rear group includes, in order from the object side, a positive single lens and one or more cemented lenses, and following conditional expressions (1), (2), (3), and (5) are satisfied. 1.95<ndCLn (1), 35<??dCL (2), 1.1<fL2/ft<1.6 (3), ?7<R1/ft<?3.6 (5).

Abstract: An objective optical system includes, in order from an object side to an image side, a front group having a negative refractive power, an aperture stop, and a rear group having a positive refractive power. The front group includes a negative single lens, the rear group includes, in order from the object side, a positive single lens and one or more cemented lenses, and following conditional expressions (1), (2), (3), and (5) are satisfied. 1.95<ndCLn (1), 35<??dCL (2), 1.1<fL2/ft<1.6 (3), ?7<R1/ft<?3.6 (5).

Abstract: An endoscope includes: a light guide fiber having an exit end through which illumination light is emitted; a transparent distal-end element configured to hold the exit end of the light guide fiber and made of a transparent material that allows the illumination light emitted from the exit end to pass therethrough; and an objective optical system inserted through a through-hole provided in the transparent distal-end element, wherein at least a portion of the illumination light enters the objective optical system through a boundary between the transparent distal-end element and the objective optical system, and a refractive index of the transparent material constituting the transparent distal-end element is larger than a refractive index of at least one material in contact with an inner surface of the through-hole.