Imaging lens
An imaging lens includes, from an object side to an image side: a first positive lens having a convex object-side surface; an aperture stop; a second negative lens as a meniscus double-sided aspheric lens having a concave object-side surface; and a third positive lens as a meniscus double-sided aspheric lens having a concave image-side surface, wherein the second lens has a diffractive optical surface on the object side, the aspheric object-side and image-side surfaces of the third lens have pole-change points off an optical axis, and conditional expressions (1) to (4) below are satisfied: 8.0< fdoe/f <26.0 (1) 20< vd 1− vd 2<40 (2) 20< vd 3− vd 2<40 (3) 0.8< ih/f <0.95 (4).
1. An imaging lens for an image sensor which includes elements arranged in order from an object side to an image side, comprising:
a first lens having positive refractive power and a convex surface on the object side;
an aperture stop;
a second lens that is a meniscus double-sided aspheric lens having negative refractive power and a concave surface on the object side; and
a third lens that is a meniscus double-sided aspheric lens having positive refractive power near an optical axis and a concave surface on the image side, wherein
a diffractive optical surface is formed on the object-side surface of the second lens;
the aspheric object-side and image-side surfaces of the third lens have pole-change points off an optical axis; and
conditional expressions (1) to (4) below are satisfied:
8.0 <fdoe/f< 26.0 (1)
20< vd 1− vd 2<40 (2)
20< vd 3− vd 2<40 (3)
0.8< ih/f< 0.95 (4)
where
fdoe: focal length of the diffractive optical surface,
f: focal length of an overall optical system of the imaging lens,
vd1: Abbe number of the first lens at d-ray,
vd2: Abbe number of the second lens at d-ray,
vd3: Abbe number of the third lens at d-ray, and
ih: maximum image height.
2. The imaging lens according to claim 1 ,
wherein on the diffractive optical surface, the number of orbicular zones in an effective diameter is 10 or less and the number of orbicular zones in an area through which rays converging on the optical axis pass is 5 or less.
3. The imaging lens according to claim 2 ,
wherein a conditional expression (5) below is satisfied:
0.2< t 3/| r 3|<0.6 (5)
where
t3: distance on the optical axis from the aperture stop to the object-side surface of the second lens, and
r3: curvature radius of the object-side surface of the second lens.
4. The imaging lens according to claim 2 ,
wherein conditional expressions (6) and (7) below are satisfied:
−1.2<( r 1+ r 2)/( r 1− r 2)<−0.6 (6)
−7.0<( r 3+ r 4)/( r 3− r 4)<−1.2 (7)
where
r1: curvature radius of the object-side surface of the first lens,
r2: curvature radius of the image-side surface of the first lens,
r3: curvature radius of the object-side surface of the second lens, and
r4: curvature radius of the image-side surface of the second lens.
5. The imaging lens according to claim 2 ,
wherein conditional expressions (8) to (10) below are satisfied:
1.0< f 1/ f< 1.5 (8)
−6.0< f 2/ f<− 1.0 (9)
0.7< f 3/ f< 2.4 (10)
where
f1: focal length of the first lens,
f2: focal length of the second lens including the diffractive optical surface,
f3: focal length of the third lens, and
f: focal length of the overall optical system of the imaging lens.
6. The imaging lens according to claim 1 ,
wherein a conditional expression (5) below is satisfied:
0.2< t 3/| r 3|<0.6 (5)
where
t3: distance on the optical axis from the aperture stop to the object-side surface of the second lens, and
r3: curvature radius of the object-side surface of the second lens.
7. The imaging lens according to claim 1 ,
wherein conditional expressions (6) and (7) below are satisfied:
−1.2<( r 1+ r 2)/( r 1− r 2)<−0.6 (6)
−7.0<( r 3+ r 4)/( r 3− r 4)<−1.2 (7)
where
r1: curvature radius of the object-side surface of the first lens,
r2: curvature radius of the image-side surface of the first lens,
r3: curvature radius of the object-side surface of the second lens, and
r4: curvature radius of the image-side surface of the second lens.
8. The imaging lens according to claim 1 ,
wherein conditional expressions (8) to (10) below are satisfied:
1.0< f 1/ f< 1.5 (8)
−6.0< f 2/ f<− 1.0 (9)
0.7< f 3/ f< 2.4 (10)
where
f1: focal length of the first lens,
f2: focal length of the second lens including the diffractive optical surface,
f3: focal length of the third lens, and
f: focal length of the overall optical system of the imaging lens.