Endoscope objective optical system

Abstract

There is provided an endoscope objective optical system with a small size and a high image quality, and in which, an adequate space for disposing a visual-field direction converting element is secured. The endoscope objective optical system, comprising in order from an object side: a first group having a negative refractive power; an aperture stop; and a second group having a positive refractive power, wherein the first group includes in order from the object side, a first lens having a negative refractive power and a visual-field direction converting member, and the second group includes in order from the object side, a second lens having a biconvex shape and a cemented lens in which a third lens having a positive refractive power and a fourth lens having a negative refractive power are cemented in this order, and the endoscope objective optical system satisfies the following conditional expressions (1), (2), (3), and (4) 2.4≦d1/f≦4.6   (1) 1.85≦f2/f≦2.6   (2) −50≦r21/r22≦−0.4   (3) 2.01≦f03/f≦2.5   (4) where, d1 denotes an air conversion length from a surface on an image side of the first lens up to a surface of the aperture stop, f denotes a focal length of the overall endoscope objective optical system, f2 denotes a focal length of the second group, r21 denotes a radius of curvature of a surface on the object side of the second lens, r22 denotes a radius of curvature of a surface on the image side of the second lens, and f03 denotes a focal length of the third lens.

Dependencies are inferred from the claim text.