Imaging lens

An imaging lens comprises a front group constituted by a first lens L.sub.1 made of a negative meniscus lens having a convex surface directed onto the object side, the object-side surface thereof being made aspheric, and a second lens L.sub.2 made of a biconvex lens having a surface with a larger curvature directed onto the object side; and a rear group constituted by a third lens L.sub.3 made of a biconcave lens and two biconvex lenses L.sub.4, L.sub.5. Letting d be the distance from the object-side surface of the first lens L.sub.1 to the image-side surface of the second lens L.sub.2, f be the focal length of the whole lens system, and n.sub.A and .nu..sub.A be average values of refractive index and Abbe number, respectively, in the two biconvex lenses L.sub.4, L.sub.5, the imaging lens satisfies 1.0<d/f<2.0, n.sub.A >1.76, and .nu..sub.A >45.0. Thus, while maintaining a compact configuration constituted by five lenses, the imaging lens allows various kinds of aberration such as chromatic aberration in magnification to become favorable even when applied to a CCD imaging device having a large number of pixels.

RELATED APPLICATIONS
 This application claims the priority of Japanese Patent Application No.
 10-276318 filed on Sep. 30, 1998, which is incorporated herein by
 reference.
 BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates, in particular, to an imaging lens employed
 in an image pickup instrument such as a compact electronic still camera or
 the like. More specifically, the present invention relates to an imaging
 lens which can also match an imaging device having a large number of
 pixels such as CCD.
 2. Description of the Prior Art
 In recent years, electronic still cameras with which pictures are easier to
 take and view than with conventional cameras using silver halide films
 have rapidly been coming into widespread use.
 There have been strong demands for imaging lenses used in this kind of
 electronic still cameras to be of a high resolution, high performances, a
 small size, and a low cost. In addition, in the electronic still camera
 using a CCD imaging device, the imaging lens is required to be
 telecentric.
 As an imaging lens satisfying such a requirement, a two-group, five-element
 configuration disclosed in Japanese Unexamined Patent Publication No.
 10-20188 has been known.
 Meanwhile, the recent development of CCD imaging devices is so remarkable
 that those having 1.5 million pixels have lately come into practice use.
 Hence, it has been demanded that such a CCD imaging device having a large
 number of pixels should be mounted to an electronic still camera, so as to
 yield images with a higher resolution.
 When the prior art disclosed in the above-mentioned publication is employed
 in an instrument with a CCD having such a large number of pixels, its
 chromatic aberration in magnification would increase in particular, so
 that inconsistencies in color may occur on a reproduced picture.
 SUMMARY OF THE INVENTION
 In order to solve such a problem, it is an object of the present invention
 to provide an imaging lens having a high resolution and a relatively far
 exit pupil, satisfying demands for a smaller size and a lower cost, and
 being capable of allowing various kinds of aberration, such as chromatic
 aberration in magnification in particular, to become favorable even when
 employed in an instrument with a CCD imaging device having a large number
 of pixels.
 The imaging lens in accordance with the present invention comprises,
 successively from an object side,
 a front group comprising, successively from the object side, a first lens
 of a meniscus form having a negative refracting power and at least one
 aspheric surface, a convex surface thereof being directed onto the object
 side, and a second lens having a positive refracting power and a surface
 with a greater curvature directed onto the a object side; and
 a rear group comprising one lens having a negative refracting power and two
 lenses each having a positive refracting power;
 the imaging lens satisfying the following conditional expression (1):
EQU 1.0&lt;d/f&lt;2.0 (1)
 where
 d is the distance from the object-side surface of the first lens to the
 image-side surface of the second lens; and
 f is the focal length of the whole lens system.
 Also, the imaging lens preferably satisfies at least one of the following
 conditional expressions (2) and (3):
EQU n.sub.A &gt;1.76 (2)
EQU .nu..sub.A &gt;45.0 (3)
 where n.sub.A and .nu..sub.A are average values of refractive index and
 Abbe number, respectively, in the two lenses each having a positive
 refracting power.
 The object-side surface of the first lens may be made aspheric. Also, both
 surfaces of the first lens may be made aspheric.

DESCRIPTION OF THE PREFERRED EMBODIMENTS
 In the following, the imaging lens in accordance with embodiments of the
 present invention will be explained specifically with reference to
 Examples 1 to 4.
 EXAMPLE 1
 FIG. 1 shows the basic lens configuration of Example 1. As shown in FIG. 1,
 the imaging lens in accordance with Example 1 comprises, successively from
 the object side, a front group comprising, successively from the object
 side, a first lens L.sub.1 of a meniscus form having a negative refracting
 power, whose convex surface is directed onto the object side, the
 object-side surface thereof being made aspheric, and a second lens L.sub.2
 made of a biconvex lens having a surface with a greater curvature directed
 onto the object side; and a rear group comprising, successively from the
 object side, a third lens L.sub.3 made of a biconcave lens and two
 biconvex lenses L.sub.4, L.sub.5. The luminous flux made incident on the
 imaging lens along its optical axis X from the object side forms an image
 on an imaging surface (the light-receiving surface of a solid-state
 imaging device) 1.
 Disposed on the image side of the fifth lens L.sub.5 is a filter portion 2
 including an infrared cutoff filter and a low-pass filter.
 In thus constructed imaging lens, the first lens L.sub.1 having a negative
 refracting power is disposed closest to the object, so as to attain a
 retro-focus type configuration, thereby achieving a wider angle. Further,
 the object-side surface of the first lens L.sub.1 is formed into a
 predetermined aspheric surface, so as to correct distortion which tends to
 shift to the minus side.
 Also, this imaging lens is configured so as to satisfy the following
 conditional expressions (1), (2), and (3):
EQU 1.0&lt;d/f&lt;2.0 (1)
EQU n.sub.A &gt;1.76 (2)
EQU .nu..sub.A &gt;45.0 (3)
 where d is the distance from the object-side surface of the first lens
 L.sub.1 to the image-side surface of the second lens L.sub.2, f is the
 focal length of the whole lens system, and n.sub.A and .nu..sub.A are
 average values of refractive index and Abbe number, respectively, in the
 two biconvex lenses L.sub.4, L.sub.5 constituting the rear group.
 When the distance between the first lens L.sub.1 and the second lens
 L.sub.2 is set to a predetermined wide gap so as to satisfy the
 above-mentioned conditional expression (1), the first lens L.sub.1 and the
 second lens L.sub.2 can have a smaller power, whereby various kinds of
 aberration, such as chromatic aberration in magnification in particular,
 can be corrected effectively. Namely, it becomes difficult to favorably
 correct various kinds of aberration such as chromatic aberration in
 magnification if the lower limit of conditional expression (1) is not
 satisfied, whereas it becomes difficult to attain compactness if the upper
 limit of conditional expression (1) is exceeded.
 Further, the object-side surface of the second lens L.sub.2 is formed as a
 convex surface having a curvature greater than that of the image-side
 surface thereof, so as to elongate the distance to the front-side focal
 position of the second lens L.sub.2, thereby making it easier to attain a
 value within the range of conditional expression (1), whereas the convex
 power of the second lens L.sub.2 is made smaller so as to suppress various
 kinds of aberration to low levels.
 Also, when the above-mentioned conditional expression (2) is satisfied,
 various kinds of aberration such as spherical aberration, coma, curvature
 of field, and the like can be corrected favorably. When the
 above-mentioned conditional expression (3) is satisfied, on the other
 hand, axial chromatic aberration can be corrected favorably.
 The upper part of the following Table 1 shows the radius of curvature R
 (mm) of each lens surface, center thickness of each lens and air space
 between neighboring lenses D (mm), refractive index N of each lens at
 d-line, and Abbe number .nu. of each lens in Example 1.
 In Table 1 and Tables 3, 5, and 7 which will be described later, the
 numbers referring to the letters R, D, N, and .nu. increase successively
 from the object side. Here, the surface indicated by "*" is an aspheric
 surface, which is represented by the following aspheric surface
 expression.
 Aspheric surface expression:
EQU X=(y.sup.2 /r)/{1+[1-K(y/r).sup.2 ].sup.1/2 }+.SIGMA.A.sub.2i Y.sup.2i
 where
 X is the length of the perpendicular to the tangential plane of an apex of
 the aspheric surface from a point on the aspheric surface at a height y
 from the optical axis;
 y is the height from the optical axis;
 r is the radius of curvature near the apex of the aspheric surface;
 K is the cone coefficient; and
 A.sub.2i is the aspheric surface coefficient of the 2i-th order (i being an
 integer from 2 to 5).
 Table 2 (follows) shows the coefficients concerning the above-mentioned
 aspheric surface.
 Further, the focal length f of the whole lens system is 6.19 mm as
 indicated in the lower part of Table 1, the F number is 2.80, the half
 angle of view .omega. is 30.3 degrees, and the value of d/f is 1.18.
 Hence, all of the above-mentioned conditional expressions (1), (2), and
 (3) are satisfied.
 EXAMPLE 2
 The imaging lens in accordance with Example 2 has a configuration
 substantially similar to that of the imaging lens in accordance with
 Example 1 shown in FIG. 1, thereby yielding similar effects, except that
 the second lens L.sub.2 is a positive meniscus lens.
 The upper part of the following Table 3 shows the radius of curvature R
 (mm) of each lens surface, center thickness of each lens and air space
 between neighboring lenses D (mm), refractive index N of each lens at
 d-line, and Abbe number .nu. of each lens in Example 2. Here, the surface
 indicated by "*" is an aspheric surface, which is represented by the
 above-mentioned aspheric surface expression.
 Table 4 (follows) shows the coefficients concerning the above-mentioned
 aspheric surface.
 Further, the focal length f of the whole lens system is 6.19 mm as
 indicated in the lower part of Table 3, the F number is 2.80, the half
 angle of view .omega. is 30.3 degrees, and the value of d/f is 1.42.
 Hence, all of the above-mentioned conditional expressions (1), (2), and
 (3) are satisfied.
 EXAMPLE 3
 As shown in FIG. 2, the imaging lens in accordance with Example 3 has a
 configuration substantially similar to that of the imaging lens in
 accordance with Example 2, thereby yielding similar effects, except that
 not only the object-side surface but also the image-side surface of the
 second lens L.sub.2 is made aspheric.
 The upper part of the following Table 5 shows the radius of curvature R
 (mm) of each lens surface, center thickness of each lens and air space
 between neighboring lenses D (mm), refractive index N of each lens at
 d-line, and Abbe number .nu. of each lens in Example 3. Here, each surface
 indicated by "*" is an aspheric surface, which is represented by the
 above-mentioned aspheric surface expression.
 Table 6 (follows) shows the coefficients concerning the above-mentioned
 aspheric surfaces.
 Further, the focal length f of the whole lens system is 6.19 mm as
 indicated in the lower part of Table 5, the F number is 2.80, the half
 angle of view .omega. is 30.2 degrees, and the value of d/f is 1.29.
 Hence, all of the above-mentioned conditional expressions (1), (2), and
 (3) are satisfied.
 EXAMPLE 4
 As shown in FIG. 3, the imaging lens in accordance with Example 4 has a
 configuration substantially similar to that of the imaging lens in
 accordance with Example 1, thereby yielding similar effects.
 The upper part of the following Table 7 shows the radius of curvature R
 (mm) of each lens surface, center thickness of each lens and air space
 between neighboring lenses D (mm), refractive index N of each lens at
 d-line, and Abbe number .nu. of each lens in Example 4. Here, the surface
 indicated by "*" is an aspheric surface, which is represented by the
 above-mentioned aspheric surface expression.
 Table 8 (follows) shows the coefficients concerning the above-mentioned
 aspheric surface.
 Further, the focal length f of the whole lens system is 6.19 mm as
 indicated in the lower part of Table 7, the F number is 2.80, the half
 angle of view .omega. is 30.3 degrees, and the value of d/f is 1.94.
 Hence, all of the above-mentioned conditional expressions (1), (2), and
 (3) are satisfied.
 FIGS. 4, 5, 6, and 7 show various kinds of aberration (spherical
 aberration, astigmatism, distortion, and chromatic aberration in
 magnification) in Examples 1, 2, 3, and 4, respectively. In these
 aberration charts, .omega. indicates the half angle of view. As can be
 seen from these charts, various kinds of aberration including chromatic
 aberration in magnification can be made favorable in accordance with the
 embodiments of the present invention.
 Here, a cover glass can also be inserted, together with the low-pass filter
 and infrared cutoff filter, between the imaging lens and the imaging
 surface (the light-receiving surface of the solid-state imaging device) 1.
 In the imaging lens in accordance with the present invention, as explained
 in the foregoing, the gap between the first and second lenses constituting
 the front group is set to a predetermined wide distance, and the
 object-side surface of the second lens is formed as a surface having a
 large curvature, whereby, while in a compact configuration made of five
 lenses, various kinds of aberration such as chromatic aberration in
 magnification can be made favorable, and good reproduced images which are
 free of problems such as inconsistencies in color and the like can be
 obtained even when the imaging lens is employed in an image pickup
 instrument mounted with a CCD imaging device having a large number of
 pixels.
 TABLE 1
 Example 1
 Surface R D N.sub.d v.sub.d
 1* 16.8252 2.25011 1.71300 53.9
 2 4.2007 3.30000
 3 12.3876 1.78230 1.84666 23.8
 4 -499.8480 5.51732
 5 -21.5487 0.91253 1.84666 23.8
 6 7.0933 2.80444 1.69680 55.5
 7 -7.9424 0.20161
 8 15.1686 2.00543 1.83500 43.0
 9 -42.4969 3.00000
 10 .infin. 4.32800 1.51633 64.1
 11 .infin.
 f = 6.19
 d/f = 1.18
 TABLE 2
 Example 1
 First Surface
 K = 1.0000000
 A.sub.4 = 0.3124700 .times. 10.sup.-3
 A.sub.6 = -0.3557808 .times. 10.sup.-6
 A.sub.8 = 0.4936034 .times. 10.sup.-7
 A.sub.10 = 0.1923878 .times. 10.sup.-9
 TABLE 3
 Example 2
 Surface R D N.sub.d v.sub.d
 1* 15.9048 2.04477 1.72000 50.3
 2 4.3923 5.00000
 3 11.5649 1.75069 1.84666 23.8
 4 78.3111 5.93685
 5 -38.0331 0.91204 1.84666 23.8
 6 6.5164 2.79182 1.71300 53.9
 7 -10.0014 0.20012
 8 14.1156 1.98059 1.83500 43.0
 9 -74.8459 3.00000
 10 .infin. 4.32800 1.51633 64.1
 11 .infin.
 f = 6.19
 d/f = 1.42
 TABLE 4
 Example 2
 First Surface
 K = 1.0000000
 A.sub.4 = 0.2874452 .times. 10.sup.-3
 A.sub.6 = 0.3831024 .times. 10.sup.-6
 A.sub.8 = 0.3341652 .times. 10.sup.-7
 A.sub.10 = 0.1626714 .times. 10.sup.-9
 TABLE 5
 Example 3
 Surface R D N.sub.d v.sub.d
 1* 16.3581 2.21428 1.71300 53.9
 2* 4.4127 3.50000
 3 8.5350 2.28690 1.84666 23.8
 4 21.4017 5.34537
 5 -24.7490 1.32603 1.84666 23.8
 6 6.9491 2.74343 1.69680 55.5
 7 -8.4993 0.20001
 8 11.7674 2.07174 1.83500 43.0
 9 -85.0060 3.00000
 10 .infin. 4.32800 1.51633 64.1
 11 .infin.
 f = 6.19
 d/f = 1.29
 TABLE 6
 Example 3
 First surface Second surface
 K = 1.0000000 K = 1.0000000
 A.sub.4 = 0.8638107 .times. 10.sup.-3 A.sub.4 = 0.1276096 .times.
 10.sup.-2
 A.sub.6 = -0.1227603 .times. 10.sup.-4 A.sub.6 = 0.6582005 .times.
 10.sup.-5
 A.sub.8 = 0.1150956 .times. 10.sup.-6 A.sub.8 = -0.1394860 .times.
 10.sup.-8
 A.sub.10 = 0.3479994 .times. 10.sup.-9 A.sub.10 = -0.5924202 .times.
 10.sup.-11
 TABLE 7
 Example 4
 Surface R D N.sub.d v.sub.d
 1* 15.0393 2.25000 1.83500 43.0
 2 5.0265 8.00000
 3 13.0974 1.78234 1.84666 23.8
 4 -1141.4483 5.83976
 5 -21.5516 1.17816 1.84666 23.8
 6 6.0387 2.83105 1.72000 50.3
 7 -10.5416 0.20000
 8 12.1764 1.98064 1.83500 43.0
 9 -12315.2709 3.00000
 10 .infin. 4.32800 1.51633 64.1
 11 .infin.
 f = 6.19
 d/f = 1.94
 TABLE 8
 Example 4
 First Surface
 K = 1.0000000
 A.sub.4 = 0.1592999 .times. 10.sup.-3
 A.sub.6 = 0.6717857 .times. 10.sup.-6
 A.sub.8 = 0.6871770 .times. 10.sup.-8
 A.sub.10 = 0.1673892 .times. 10.sup.-9