Patent ID: 8625208

Claim:
An image pickup optical system comprising, in order from an object side, a first lens having a positive refractive power, a second lens having a negative refractive power, a third lens having a positive refractive power, a fourth lens having a positive refractive power, a fifth lens having a negative refractive power, which is formed into a meniscus shape on an optical axis and having its concave surface facing toward the object side, and an aperture located on the most object side, wherein the following conditional expression (5) is satisfied: 1<( r 8 +r 9)/( r 8 −r 9)<2.7 (5) where r8 is a paraxial radius of curvature of an object side surface of the fourth lens, and r9 is a paraxial radius of curvature of an image side surface of the fourth lens.