Imaging lens
An imaging lens includes four lenses arranged in order from the object side to the image side: an aperture stop, positive (refractive power) first lens having a convex object-side surface near the optical axis, second lens having a positive meniscus shape near the axis, positive third lens having a convex image-side surface near the axis, and negative fourth lens having a concave image-side surface near the axis. All lens surfaces are aspheric. The image-side aspheric surface of the fourth lens has a pole-change point off the optical axis and conditional expressions (1) and (2) are satisfied: 0.75< TLA /(2 IH )<0.90 (1) 0.90< TLA/f <1.30 (2) where TLA: distance on the optical axis from the first lens's object-side surface to the image sensor's image plane without a filter between the fourth lens and image sensor IH: maximum image height f: overall optical system focal length.
1. An imaging lens for a solid-state image sensor in which lenses are arranged in order from an object side to an image side, comprising:
a first lens with positive refractive power having a convex surface on the object side near an optical axis;
a second lens having a positive meniscus shape near the optical axis;
a third lens with positive refractive power having a convex surface on the image side near the optical axis; and
a fourth lens with negative refractive power having a concave surface on the image side near the optical axis,
wherein both surfaces of each lens are aspheric;
wherein the image-side aspheric surface of the fourth lens has a pole-change point in a position off the optical axis; and
wherein conditional expressions (1) and (2) below are satisfied:
0.75 <TLA /(2 IH )<0.90 (1)
0.90 <TLA/f< 1.30 (2)
where
TLA: distance on the optical axis from the object-side surface of the first lens to an image plane of the image sensor when a filter between the fourth lens and the image sensor is removed
IH: maximum image height
f: focal length of an overall optical system of the imaging lens.
2. The imaging lens according to claim 1 , wherein a conditional expression (3) below is satisfied:
0.4 <f 1 /f 3<5.5 (3)
where
f1: focal length of the first lens
f3: focal length of the third lens.
3. The imaging lens according to claim 1 , wherein conditional expressions (4) and (5) below are satisfied:
50≦ νd≦ 70 (4)
ν d 1 =νd 2 =νd 3 =νd 4 (5)
where
νd: Abbe number of the first lens to the fourth lens
νd1: Abbe number of the first lens
νd2: Abbe number of the second lens
νd3: Abbe number of the third lens
νd4: Abbe number of the fourth lens.
4. The imaging lens according to claim 1 , wherein a conditional expression (6) below is satisfied:
−20.0<( r 1 +r 2)/( r 1 −r 2)<−0.50 (6)
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.
5. The imaging lens according to claim 1 , wherein a conditional expression (7) below is satisfied:
1.00 <f 123 /|f 4|<1.20 (7)
where
f123: composite focal length of the first, second, and third lenses
f4: focal length of the fourth lens.
6. An image pickup device comprising the imaging lens according to claim 1 and an image sensor of ⅕ inch or less whose number of pixels is 5 megapixels or less.
7. The imaging lens according to claim 1 , wherein a diffractive optical surface is provided on at least one of surfaces from the object-side surface of the first lens to the image-side surface of the second lens.
8. The imaging lens according to claim 2 , wherein conditional expressions (4) and (5) below are satisfied:
50 ≦νd≦ 70 (4)
ν d 1 =νd 2 =νd 3 =νd 4 (5)
where
νd: Abbe number of the first lens to the fourth lens
νd1: Abbe number of the first lens
νd2: Abbe number of the second lens
νd3: Abbe number of the third lens
νd4: Abbe number of the fourth lens.
9. The imaging lens according to claim 2 , wherein a conditional expression (6) below is satisfied:
−20.0<( r 1 +r 2)/( r 1 −r 2)<−0.50 (6)
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.
10. The imaging lens according to claim 2 , wherein a conditional expression (7) below is satisfied:
1.00 <f 123 /|f 4|<1.20 (7)
where
f123: composite focal length of the first, second, and third lenses
f4: focal length of the fourth lens.
11. An image pickup device comprising the imaging lens according to claim 2 and an image sensor of ⅕ inch or less whose number of pixels is 5 megapixels or less.
12. The imaging lens according to claim 2 , wherein a diffractive optical surface is provided on at least one of surfaces from the object-side surface of the first lens to the image-side surface of the second lens.