Objective optical system for endoscope

An objective optical system for an endoscope, the objective optical system consisting of, in order from an object side:

• a first lens having a negative refractive power;
• a second meniscus lens having a positive refractive power and having a convex surface directed toward an image side;
• an aperture stop;
• a third lens having a positive refractive power;
• and
• a cemented lens of a fourth lens having a positive refractive power and a fifth lens having a negative refractive power,
• wherein the objective optical system satisfies the following conditional expressions (1-1), (1-2), and (1-3)
• −0.6≤f1/f45≤−0.18 (1-1)
• 0.2≤(r3f+r3r)/(r3f−r3r)≤1 (1-2)
• 0.15≤d34/d4≤0.7 (1-3)
• where,
• f1 denotes a focal length of the first lens,
• f45 denotes a combined focal length of the fourth lens and the fifth lens,
• r3f denotes a radius of curvature of an object side of the third lens,
• r3r denotes a radius of curvature of an image side of the third lens,
• d34 denotes a distance along an optical axis between the third lens and the fourth lens, and
• d4 denotes a thickness of the fourth lens.