Patent ID: 8531781

Claim:
An objective optical system comprising, in order from an object side, a negative-powered first group, an aperture stop, a positive-powered second group, and a third group, wherein: a surface of the first group which is closest to an image side is a concave surface turned toward the image side; a surface of the second group which is closest to the image side is an aspheric surface; a surface of the third group which is closest to an object side is a convex surface turned toward the object side; the objective optical system has a three-group, three-lens configuration in which each group is made up of a single lens, all surfaces of the three lenses have power and are configured to bend a beam by refraction, and all the three lenses of the first to third groups are optical devices made of a material with a refractive index of 1.65 or less; and a distance between the second group and the third group is smaller than a distance between the first group and the second group, the objective optical system satisfying a conditional expression below and having an angle of view of 130 degrees or more d — L 23 /d — L 12<0.25 1.7 <f — L 3 /f — L 2 where d_L 23 is the distance between the second group and the third group, d_L 12 is the distance between the first group and the second group, f_L 2 is a focal length of the second group, and f_L 3 is a focal length of the third group.