Patent ID: 8767307

Claim:
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: −0.5 <f/f 4<−0.001, and (7) 0.1 ≦|f 4 /f 2|≦5, where f is a focal length of an entire system, f 4 is a focal length of the fourth group, and f 2 is a focal length of the second group.