Optical device

An optical device comprising:

• an image forming lens disposed in an optical path between an objective lens and an imaging device, the image forming lens forming an image of light incident via the objective lens on an imaging surface of the imaging device;
• a beam splitter disposed in an optical path between the objective lens and the image forming lens;
• a first lens group disposed in an optical path between the objective lens and the beam splitter and having a positive refractive power;
• and
• a coaxial epi-illumination optical system configured to be able to illuminate an imaging target via the beam splitter, the first lens group, and the objective lens;
• the coaxial epi-illumination optical system being arranged in an optical path different from the optical path in which the image forming lens is arranged, wherein
• the first lens group consists of:
• a first lens having a positive refractive power;
• and
• a second lens having a negative refractive power.