Patent ID: 7602560

Claim:
An image pickup lens for forming an image of an object on a photoelectrical converter of a solid-state image pickup element, the image pickup lens comprising, in order from an object side thereof: an aperture stop; a first lens having a positive refractive power and having a meniscus shape whose surface facing the object side is convex; a second lens having a negative refractive power whose surface facing an image side of the image pickup lens is concave; and a third lens having a positive refractive power and having a meniscus shape whose surface facing the object side is convex, wherein each of the first lens, the second lens, and the third lens comprises an aspheric optical surface, and the image pickup lens satisfies following expressions: 0 <P air/ P < 1.0 0.7 <f 1/ f< 1.25 where f is a focal length of the total system of the image pickup lens, f 1 is a focal length of the first lens, P is a refractive power of a total system of the image pickup lens, and Pair is a refractive power of an air lens which is formed from the surface of the second lens facing the image side and the surface of the third lens facing the object side, and which is defined by a following expression: Pair = 1 - n ⁢ ⁢ 2 R ⁢ ⁢ 4 + n ⁢ ⁢ 3 - 1 R ⁢ ⁢ 5 - D ⁢ ⁢ 23 · ( 1 - n ⁢ ⁢ 2 ) · ( n ⁢ ⁢ 3 - 1 ) R ⁢ ⁢ 4 · R ⁢ ⁢ 5 , where n 2 is a refractive index of the second lens for d line, n 3 is a refractive index of the third lens for d line, R 4 is a curvature radius of the surface of the second lens facing the image side, R 5 is a curvature radius of the surface of the third lens facing the object side, and D 23 is a length of an air space between the second lens and the third lens along an optical axis of the image pickup lens.