Patent ID: 8582217

Claim:
An optical system for an endoscope, the optical system comprising: a negative lens group, an aperture diaphragm and a positive lens group in this order from an object side, wherein the negative lens group includes a negative first lens and a negative second lens in this order from the object side; and wherein the optical system meets conditional expressions (1) and (2): 0.05 <H (100)×( nd 01−1)/ Rf 01<0.38 (1) and 0.5 <enp /FL<2.5 (2), where: H(100) is a height at which a principal ray with a field angle of incidence of 100 degrees falls on a surface on the object side of the first lens; Rf01 is a curvature radius of the surface on the object side of the first lens; nd01 is a refractive index of the first lens; FL is a focal length of an entire system; and enp is a distance between an entrance pupil and the surface on the object side of the first lens.