An imaging lens having a two-group, two-lens-element construction is disclosed that is formed of, in order from the object side without any intervening lens elements: an aperture stop; a first lens element of positive refractive power with its surface on the object side being aspherical and convex near the optical axis and its surface on the image side being aspherical and either convex or concave near the optical axis; and a second lens element of positive refractive power with its surface on the object side being aspherical and convex near the optical axis and its surface on the image side being aspherical and concave near the optical axis but convex toward the periphery. Specified conditions are satisfied in order to reduce various aberrations and provide a sufficient back focus for the insertion of other optical elements.
1. An imaging lens having a two-group, two-lens-element construction, said imaging lens comprising, in order from the object side without any intervening lens element:
an aperture stop;
a first lens element of positive refractive power with its surface on the object side being aspherical and convex near the optical axis and its surface on the image side being aspherical and either convex or concave near the optical axis; and
a second lens element of positive refractive power with its surface on the object side being aspherical and convex near the optical axis and its surface on the image side being aspherical and concave near the optical axis but convex toward the periphery;
wherein the following Conditions (1) and (2) are satisfied:
0.6 ≦D 3 /D 2 ≦1.0 Condition (1)
υ 1 , υ 2 ≧50 Condition (2)
where
D 3 is the central thickness of the second lens element,
D 2 is the distance on the optical axis between the image-side surface of the first lens element and the object-side surface of the second lens element,
υ 1 is the Abbe number of the first lens element, and
υ 2 is the Abbe number of the second lens element.
2. The imaging lens of claim 1 , wherein the following Condition (3) is satisfied:
1.48≦ Bf/D 2 ≦2.3 Condition (3)
where
Bf is the back focal length of the imaging lens.
3. The imaging lens of claim 2 , wherein the aspherical shape of the object-side surface and the image-side surface of the second lens element are defined using at least one odd-order coefficient Ai in the following Equation (A):
Z=Cρ 2 /(1+(1 −K C 2 ρ 2 ) 1/2 )+Σ A i ·ρ i Equation (A)
where
Z is the length of a perpendicular line drawn from a point on an aspherical surface at a height ρ from the optical axis to the contact plane (a plane perpendicular to the optical axis) of the apex of the aspherical surface,
ρ is the distance from the optical axis,
K is a constant,
C is the paraxial curvature of the aspherical surface (=1/R, where R is the radius of curvature of the aspherical surface on-axis), and
A i is the i th order aspherical coefficient, where i equals 3 through 10.
4. The imaging lens of claim 1 , wherein the aspherical shape of the object-side surface and the image-side surface of the second lens element are defined using at least one odd-order coefficient Ai in the following Equation (A):
Z=Cρ 2 /(1+(1 −K C 2 ρ 2 ) 1/2 )+Σ A i ·ρ i Equation (A)
where
Z is the length of a perpendicular line drawn from a point on an aspherical surface at a height ρ from the optical axis to the contact plane (a plane perpendicular to the optical axis) of the apex of the aspherical surface,
ρ is the distance from the optical axis,
K is a constant,
C is the paraxial curvature of the aspherical surface (=1/R, where R is the radius of curvature of the aspherical surface on-axis), and
A i is the i th order aspherical coefficient, where i equals 3 through 10.