Objective optical system, image pickup apparatus, and endoscope

An image pickup apparatus comprising an objection optical system, the objective optical system comprising, in order from the object side:

• a flat plate;
• an aperture stop;
• and
• a substantially hemispherical planoconvex lens having an outer diameter substantially equal to the outer diameter of the flat plate,
• wherein the flat plate and the planoconvex lens are cemented together with the aperture stop between, and the objective optical system satisfies the following conditional expressions (1), (2), and (3):
• 0.3<IH/Dp<0.45 (1)
• 1.7<1+Dp×ωr/(2×IH)<2.1 (2)
• 0.9<NL/(1+Dp×ωr/(2×IH))<1.1 (3)
• where ωr=2×π×|ω|/360°, IH is the largest image height, Dp is the outer diameter of the flat plate, ω is the half angle of view, and NL is the refractive index of the planoconvex lens.

An endoscope comprising an objection optical system, the objective optical system comprising, in order from the object side:

• a flat plate;
• an aperture stop;
• and
• a substantially hemispherical planoconvex lens having an outer diameter substantially equal to the outer diameter of the flat plate,
• wherein the flat plate and the planoconvex lens are cemented together with the aperture stop between, and the objective optical system satisfies the following conditional expressions (1), (2), and (3):
• 0.3<IH/Dp<0.45 (1)
• 1.7<1+Dp×ωr/(2×IH)<2.1 (2)
• 0.9<NL/(1+Dp×ωr/(2×IH))<1.1 (3)
• where ωr=2×π×|ω|/360°, IH is the largest image height, Dp is the outer diameter of the flat plate, ω is the half angle of view, and NL is the refractive index of the planoconvex lens.