JP2007193195A - Photographic lens and imaging apparatus having the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain a compact photographic lens constituted of a comparatively small number of lenses, and having high optical performance. <P>SOLUTION: The photographic lens has: an aperture diaphragm; a first lens having positive refractive power, whose surfaces on an object side and an image side are convex; a second lens having negative refractive power, whose surface on the image side has convex and meniscus shape; a third lens having positive refractive power, whose surfaces on the object side and the image side are convex; and a fourth lens having negative refractive power in order from the object side to the image side. At least one surface of each lens is aspherical, and the material of each lens is resin. The photographic lens satisfies conditions N1<1.54 and ν1>ν2 when the refractive index and the Abbe number of the material of the first lens are defined as N1 and ν1 respectively, and the Abbe number of the material of the second lens is defined as ν2. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention is a high-performance and compact device suitable for a small-sized image pickup apparatus using a solid-state image pickup device such as a CCD such as a mobile phone such as a digital still camera (hereinafter referred to as DSC), a surveillance camera, and an in-vehicle camera. The present invention relates to a taking lens.

  In recent years, DSC has been rapidly improved in functionality and image quality.

  Until recently, DSC has been pointed out to improve resolution, dynamic range, and the like with respect to a solid-state imaging device such as a CCD because the resolution of a photographed image is lower than that of a camera for a silver salt film.

  However, recently, with the development of a drive circuit and a signal amplifier circuit with high frequency characteristics, DSC has become approximately the same as that of a camera for silver salt film, for example, in terms of resolution. DSC is easy to automate focus (focus), light intensity (aperture, shutter), white balance (color temperature), and with techniques such as camera shake correction and backlight correction, high-quality images can be taken easily. It has become.

  Recently, in a DSC, a color filter for obtaining a color image, a microlens for increasing sensitivity, and the like are placed immediately before a light receiving surface of a CCD.

  In general, the light beam is obliquely incident on the periphery of the light-receiving surface from the photographic lens. Therefore, the image is falsely colored due to the optical action of the color filter and microlens at the center of the screen and the periphery of the screen. May happen.

  The exit pupil condition (telecentric condition) is a condition for the photographing lens corresponding to the angle of the light beam obliquely incident on the peripheral portion of the CCD screen. This condition is expressed by the distance in the optical axis direction from the CCD at the exit pupil position or the incident angle of the principal ray on the CCD surface.

  It is necessary that the conditions of the incident light match the layout of a microlens or the like provided in the CCD, and there is an exit pupil distance or chief ray incident angle recommended for each CCD.

  Conventionally, since the limit of the incident angle is strict and it is within 8 to 10 degrees in the entire screen, it is very difficult to reduce the size of the photographing lens.

  However, recent improvements in CCDs have allowed the incident angle to be about 20 ° in DSC and about 25 to 30 ° in mobile phones, making it easy to reduce the size of the taking lens.

  As a small photographic lens, there has conventionally been proposed a photographic lens that is suitable for DSC, has good telecentricity, and has about four lenses.

  Among these, a photographing lens composed of four groups of four elements of an aperture stop, a positive lens, a negative lens, a positive lens, and a negative lens in order from the object side to the image side is known (Patent Document 1).

As a photographic lens, a small photographic lens composed of four groups and four lenses using a lens made of resin is known from the viewpoint of ease of manufacturability (Patent Document 2).
JP 2002-365530 A JP-A-2005-208236

  In an imaging apparatus using a solid-state imaging device, if the distance from the image plane to the exit pupil is extremely short, the incident angle of off-axis rays on the light receiving surface (image plane) increases.

  When light rays are incident on the image plane at an angle, the aperture efficiency is reduced due to shading. For this reason, it is desired that the photographing lens used for DSC has good telecentricity on the image side (the exit pupil position is sufficiently away from the image plane).

  In particular, in an imaging apparatus using a solid-state imaging device, a large amount of shading occurs when the angle formed by the on-axis principal ray and the off-axis principal ray exceeds a certain value.

  For this reason, a lens system used in an imaging apparatus using a solid-state imaging device needs to have an angle between an off-axis principal ray and an axial principal ray from the final lens to the maximum image height. Yes.

  In order to make a telecentric optical system, a lens configuration in which the exit pupil is extremely far away from the image plane often has a long overall lens length and an extremely strong refractive power of the lens, which is a compact and high-performance optical system. It becomes difficult to compose.

  On the other hand, in order to make the camera small and thin with emphasis on portability, it is necessary to shorten the total lens length of the photographing lens. In order to shorten the total lens length, it is necessary to reduce the number of constituent lenses as much as possible.

  However, simply reducing the number of lenses makes it difficult to obtain high optical performance. In view of the ease of manufacturing the photographic lens, it is more effective to use a resin material than a glass material for the lens material.

  However, since the refractive index of the resin material is generally low, it is difficult to correct aberrations properly unless the lens shape is set appropriately.

  SUMMARY OF THE INVENTION An object of the present invention is to provide a photographic lens and an image pickup apparatus having the photographic lens that are easy to manufacture while ensuring a sufficient telecentric characteristic, the entire lens system being small in size and good in optical performance.

The photographic lens of the present invention includes, in order from the object side to the image side, an aperture stop, a biconvex first lens having a positive refractive power, a second lens having a convex meniscus shape on the image side and a biconvex shape. A third lens having a positive refractive power and a fourth lens having a negative refractive power. At least one surface of each lens is aspherical, and the material of each lens is a resin. When the refractive index and Abbe number of the material are N1 and ν1, respectively, and the Abbe number of the material of the second lens is ν2,
N1 <1.54
ν1> ν2
It is characterized by satisfying the following conditions.

  According to the present invention, it is possible to achieve an imaging lens having a small size, good optical performance, and easy manufacture while securing sufficient telecentric characteristics, and an imaging apparatus having the same.

  Hereinafter, embodiments of the photographing lens of the present invention and an image pickup apparatus having the same will be described.

  1 and 2 are a lens cross-sectional view and aberration diagrams of the photographing lens of Example 1 of the present invention.

  3 and 4 are a lens cross-sectional view and aberration diagrams of the photographing lens of Example 2 of the present invention.

  5 and 6 are a lens cross-sectional view and aberration diagrams of the photographing lens of Example 2 of the present invention.

  FIG. 7 is a schematic diagram of a main part of a digital still camera (imaging device) having the photographing lens of the present invention. The photographic lens of each embodiment is a lens system used in an imaging apparatus. In the lens cross-sectional view, the left side is the object side (front), and the right side is the image side (rear).

  In the lens cross-sectional view, OB is a photographing lens.

  G1 is a first lens having positive refractive power (optical power = reciprocal of focal length) having convex surfaces on the object side and the image side.

  G2 is a second lens having a negative refractive power having a meniscus shape with a convex surface on the image side.

  G3 is a third lens having positive refractive power and convex surfaces on the object side and the image side.

  G4 is a fourth lens having a negative refractive power having a convex meniscus surface on the object side.

  Each lens has a shape based on the paraxial radius of curvature. STO is an aperture stop and is located on the object side of the first lens L1.

  i (i = 0 to 10) indicates the order of surfaces (including the aperture stop STO) from the object side. Ri (i = 1 to 8) is the lens surface of each lens, and in the numerical examples described later, the value of the radius of curvature is shown. Di is a distance between the i-th surface and the i + 1-th surface. G is an optical block corresponding to an optical filter (a crystal low-pass filter, an infrared cut filter, etc.), a face plate, and the like.

  IP is an image plane, and corresponds to an imaging plane of a solid-state imaging device (photoelectric conversion device) such as a CCD sensor or a CMOS sensor when used as a photographing lens system of a video camera or a digital still camera.

  In each embodiment, each lens is made of a resin material. The object-side and image-side surfaces of each lens are aspherical. This facilitates aberration correction. Each lens only needs to have at least one aspherical surface, which facilitates aberration correction.

  In the aberration diagrams, d, g, C, and F represent d-line, g-line, C-line, and F-line, respectively. ΔM and ΔS represent a meridional image surface and a sagittal image surface, and ω represents a half angle of view.

  Focusing from an infinitely distant object to a close object is performed by extending the aperture stop STO and the entire lens system to the object side.

  Next, the features of each embodiment will be described sequentially.

When the refractive index and Abbe number of the material of the first lens G1 are N1 and ν1, respectively, and the Abbe number of the material of the second lens G2 is ν2, N1 <1.54 (1)
ν1> ν2 (2)
Is satisfied.

  Conditional expression (1) relates to the refractive index of the material of the first lens G1, and by using a resin material that satisfies the conditional expression (1), the first lens G1 having convex surfaces on the object side and the image side is manufactured. Making it easy.

  Conditional expression (2) relates to the Abbe number of the material of the first and second lenses G1 and G2, and corrects chromatic aberration by using a material having an Abbe number larger than that of the material of the second lens G2 for the first lens G1. Have done well.

The air conversion distance (back focus) from the image side surface of the fourth lens G4 to the image surface is bf, the air conversion distance from the aperture stop STO to the image surface is TL, and the image side of the fourth lens G4 from the aperture stop STO is When the distance to the surface is ML and the focal length of the entire lens system is f,
bf / TL> 0.2 (3)
0.8 <ML / f <1.2 (4)
Is satisfied.

  If the back focus bf becomes too small beyond the lower limit of conditional expression (3), it is not possible to secure a space for inserting a cover glass or an optical low pass filter (OLPF). Furthermore, the exit pupil distance is shortened, the angle of light incident on the image plane is increased, and it is easy to be affected by shading.

  Conditional expression (4) is a conditional expression regarding the module length. When the upper limit of conditional expression (4) is exceeded, correction of spherical aberration and chromatic aberration becomes easy, but it becomes difficult to reduce the size of the entire lens system. On the contrary, if the lower limit is exceeded, correction of chromatic aberration becomes difficult and coma becomes worse, which is not good.

When the focal lengths of the first and second lenses G1 and G2 are f1 and f2, respectively 0.80 <f1 / f <1.00 (5)
0.60 <| f2 | / f <0.80 (6)
Is satisfied.

  Conditional expression (5) relates to the shape of the refractive power of the first lens G1.

  If the upper limit of conditional expression (5) is exceeded, the refractive power of the first lens G1 will become excessive, and large spherical aberration and chromatic aberration will occur.

  On the other hand, if the lower limit is exceeded, it is advantageous for correcting monochromatic aberrations, but the overall length of the lens system becomes large and it is difficult to make it compact.

  Conditional expression (6) relates to the refractive power of the second lens G2. The value of the negative refractive power of the second lens G2 needs to be set appropriately in order to correct chromatic aberration and spherical aberration that occur in the first lens G1 and the third lens G3, which have positive refractive power.

  If the upper limit of conditional expression (6) is exceeded, correction of chromatic aberration will be excessive, and it will be difficult to reduce the size of the entire lens system.

  On the contrary, if the lower limit is exceeded, correction of chromatic aberration becomes insufficient, and correction of spherical aberration and coma aberration becomes difficult.

  Further, by satisfying conditional expressions (5) and (6), a lens system with good telecentricity is obtained.

When the radius of curvature of the object-side lens surface of the second lens G2 is r3 and the radius of curvature of the object-side lens surface of the third lens G3 is r5, 0.30 <| r3 | / f <0.50. (7)
0.80 <r5 / f <1.20 (8)
Is satisfied.

  Conditional expression (7) relates to the lens shape of the second lens G2. When the upper limit of conditional expression (7) is exceeded, spherical aberration deteriorates and the influence of coma aberration becomes significant in chromatic aberration. If the lower limit is exceeded, many coma aberrations occur and the optical performance deteriorates.

  Conditional expression (8) relates to the lens shape of the third lens G3. If the upper limit is exceeded, the curvature of field greatly deteriorates in the minus direction, and the influence of halo becomes significant in chromatic aberration. Further, if the lower limit is exceeded, the aberration deteriorates in the direction opposite to the upper limit, which is not good.

When the distance from the aperture stop STO to the object side surface of the first lens G1 is D0, D0 / f> 0.10 (9)
Is satisfied.

  Conditional expression (9) relates to the distance necessary to insert a mechanical shutter on the object side of the first lens G1 and to obtain good telecentricity. By providing a mechanical shutter, light is physically blocked, and the image sensor is prevented from being deteriorated by preventing an excessive amount of light from hitting the image sensor. At the same time, noise can be reduced.

  A sufficient telecentric lens system is achieved while facilitating insertion of the mechanical shutter by ensuring a sufficient distance from the aperture stop STO to the object side of the first lens G1 so as to satisfy the conditional expression (9).

  In the imaging lens of each embodiment, when the aperture diameter is reduced, the optical performance is significantly deteriorated due to the diffraction phenomenon. For this reason, when the subject is bright, it is preferable to attach an ND filter so that it can be inserted and removed from the optical path so that the F number of the photographing lens does not become too large.

  In each embodiment, the numerical ranges of conditional expressions (1) to (9) are more preferably set as follows.

N1 <1.535 (1a)
ν1> 1.5 · ν2 (2a)
bf / TL> 0.25 (3a)
0.9 <ML / f <1.1 (4a)
0.85 <f1 / f <0.97 (5a)
0.65 <| f2 | / f <0.75 (6a)
0.35 <| r3 | / f <0.45 (7a)
0.90 <r5 / f <1.16 (8a)
D0 / f> 0.12 (9a)
In each of the above embodiments, a lens having a small refractive power may be added to the object side of the first lens G1 and / or the image side of the fourth lens G4.

  According to each embodiment, although the number of constituent lenses is small and the entire lens system is compact, a high-performance and easy-to-manufacture photographing lens can be obtained. Further, by disposing the aperture stop closest to the object side, there is a structural feature that makes the lens system inconspicuous when viewed from the object side.

  Furthermore, since it has high performance and the entire lens system is compact, it is suitable for use in a small photographing apparatus.

  Further, as described above, by taking the lens shapes of the first to fourth lenses G1 to G4 and the conditional expressions (3) and (9), an imaging lens with good telecentricity is obtained.

  The numerical examples 1 to 3 corresponding to the first to third embodiments of the present invention will be shown below. In each numerical example, i indicates the order of the surfaces from the object side, Ri is the radius of curvature of each surface, Di is the member thickness or air space between the i-th surface and the (i + 1) -th surface, Ni, ν i represents the refractive index and Abbe number for the d-line, respectively.

The two surfaces closest to the image are glass blocks G. The aspherical shape is the height h from the optical axis
Where X is the displacement in the direction of the optical axis at the position of

It is represented by Where R is a paraxial radius of curvature, k is a conic constant, and A, B, C, D, and E are aspherical coefficients.

“E-0X” means “× 10 ^−X ”. f is a focal length, Fno is an F number, ω is a half angle of view, and bf is a back focus. Table 1 shows the relationship between the above-described conditional expressions and numerical values in the numerical examples.

Numerical example 1
Example 1
f 5.712
Fno 3.5
2ω 61.5
bf 2.68


Surface No. RDN ν
STO (Aperture) ∞ 0.720
1 3.36599 1.300 1.5300 55.9
2 -17.40477 0.500
3 -2.38903 0.724 1.5855 30.0
4 43477 0.100
5 6.56010 1.574 1.5300 55.9
6 -2.05744 0.100
7 3.53596 1.051 1.5300 55.9
8 1.41508 1.210
9 ∞ 0.690 1.5168 64.2
10 ∞ 1.012

Aspherical coefficient 1st surface K = -0.790073 A = -3.27139E-03 B = 3.72297E-03
C = -4.61151E-03 D = 2.31462E-03 E = -5.36359E-04
2nd side K = 64.485389 A = -2.02480E-02 B = -1.69249E-03
C = -4.82393E-03 D = 1.56128E-03 E = -2.11052E-04
3rd surface K = 0.129172 A = 8.12431E-04 B = -3.55268E-03
C = -3.36970E-03 D = 9.91749E-04 E = 1.03755E-05
4th surface K = -0.242976E14 A = -2.10068E-03 B = -3.14650E-04
C = -3.03485E-04 D = -1.37330E-05 E = 2.36487E-06
5th surface K = 3.739393 A = -4.39385E-03 B = -4.44742E-04
C = -7.95994E-05 D = 1.28469E-05 E = -3.48245E-06
6th surface K = -2.686347 A = -1.07631E-04 B = 6.50231E-04
C = 9.35934E-05 D = 4.78264E-06 E = 8.53142E-06
7th surface K = -12.774900 A = -1.65944E-02 B = -6.62897E-04
C = 1.90354E-04 D = 6.04888E-05 E = -7.01547E-06
8th surface K = -3.388239 A = -1.75570E-02 B = 1.74511E-03
C = -5.83296E-05 D = -3.00199E-07 E = -4.11635E-08

Numerical example 2
Example 2
f 5.711
Fno 3.5
2ω 61.5
bf 2.67


Surface No. RDN ν
STO (Aperture) ∞ 0.720
1 3.42488 1.300 1.5300 55.9
2 -13.39253 0.513
3 -2.30610 0.724 1.5855 30.0
4 870131 0.100
5 6.46139 1.608 1.5300 55.9
6 -1.97266 0.100
7 3.45765 1.010 1.5300 55.9
8 1.34494 1.217
9 ∞ 0.690 1.5168 64.2
10 ∞ 1.000


Aspheric surface 1st surface K = -0.807425 A = -3.14154E-03 B = 2.56293E-03
C = -4.04431E-03 D = 2.04099E-03 E = -4.62191E-04
Second side K = 50.961982 A = -1.96355E-02 B = -1.38202E-03
C = -4.82704E-03 D = 1.53782E-03 E = -1.36313E-04
3rd surface K = 0.060327 A = 1.05900E-03 B = -2.97126E-03
C = -3.28167E-03 D = 1.02882E-03 E = 3.96662E-05
4th surface K = -0.980764E19 A = -1.16039E-03 B = -3.48792E-04
C = -2.87951E-04 D = -7.75874E-06 E = 2.79060E-06
5th surface K = 4.190301 A = -4.29705E-03 B = -3.01845E-04
C = -5.71921E-05 D = 1.97528E-05 E = -1.78677E-06
6th surface K = -2.791550 A = -7.71678E-04 B = 7.06330E-04
C = 1.34399E-04 D = 1.29449E-05 E = 9.55996E-06
7th surface K = -13.980042 A = -1.67681E-02 B = -6.74702E-04
C = 1.95564E-04 D = 6.20491E-05 E = -6.88396E-06
8th surface K = -3.497806 A = -1.73250E-02 B = 1.72395E-03
C = -6.18347E-05 D = -3.80972E-07 E = 5.06294E-08

Numerical Example 3
Example 3
f 5.710
Fno 3.5
2ω 61.5
bf 2.77


Surface No. RDN ν
STO (Aperture) ∞ 0.720
1 3.41591 1.300 1.5300 55.9
2 -15.09078 0.500
3 -2.30572 0.724 1.5855 30.0
4 -236529 0.100
5 6.50758 1.549 1.5300 55.9
6 -2.24712 0.100
7 2.85616 1.000 1.5300 55.9
8 1.39445 1.317
9 ∞ 0.690 1.5168 64.2
10 ∞ 1.000

Aspheric surface 1st surface K = -0.861977 A = -2.96673E-03 B = 3.30303E-03
C = -4.18785E-03 D = 2.34862E-03 E = -5.59514E-04
2nd surface K = 38.578174 A = -1.74403E-02 B = 6.34287E-04
C = -4.21635E-03 D = 1.69737E-03 E = -1.85545E-04
3rd surface K = -0.323243 A = 9.43014E-03 B = -1.18937E-03
C = -2.75970E-03 D = 1.04230E-03 E = 1.26720E-05
4th surface K = -0.741781E-16 A = 2.41592E-03 B = 6.93275E-05
C = -2.74475E-04 D = -4.59681E-06 E = 5.18916E-06
5th surface K = 3.738054 A = -4.63153E-03 B = -7.09457E-04
C = -7.41587E-05 D = 2.49171E-05 E = 5.78896E-07
6th surface K = -3.388744 A = -2.00306E-04 B = 1.34418E-04
C = -1.61083E-06 D = 6.53255E-06 E = 1.21241E-05
7th surface K = -5.242881 A = -1.47356E-02 B = -8.83803E-04
C = 1.42003E-04 D = 5.76310E-05 E = -5.73199E-06
8th surface K = -2.890194 A = -1.75329E-02 B = 1.76527E-03
C = -7.37248E-05 D = -4.91887E-07 E = 2.45820E-07

  Next, an embodiment of a digital still camera using a photographing lens as shown in the first to third embodiments as a photographing optical system will be described with reference to FIG.

  In FIG. 7, reference numeral 20 denotes a camera body, and 21 denotes a photographic optical system constituted by any of the photographic lenses described in the first to third embodiments.

  Reference numeral 22 denotes a solid-state imaging device (photoelectric conversion device) such as a CCD sensor or a CMOS sensor that receives a subject image formed by the photographing optical system 21 and is built in the camera body. A memory 23 records information corresponding to a subject image photoelectrically converted by the solid-state imaging device 22. Reference numeral 24 denotes a finder for observing a subject image formed on the solid-state image sensor 22, which includes a liquid crystal display panel or the like.

  Thus, by applying the photographic lens of the present invention to an image pickup device such as a digital still camera, a small-sized image pickup apparatus having high optical performance can be realized.

Lens configuration diagram of Example 1 Various aberration diagrams of Example 1 Lens configuration diagram of Example 2 Various aberration diagrams of Example 2 Lens configuration diagram of Example 3 Various aberration diagrams of Example 3 Schematic diagram of main parts of an embodiment of an imaging apparatus of the present invention

Explanation of symbols

G1 1st lens G2 2nd lens G3 3rd lens G4 4th lens STO Aperture IP image surface d d line g g line C C line F F line ΔS sagittal image surface ΔM meridional image surface G glass block

Claims (7)

In order from the object side to the image side, an aperture stop, a biconvex first lens having a positive refractive power, a second lens having a convex meniscus negative refractive power on the image side, and a first birefringent positive refractive power. 3 lenses, a fourth lens having negative refractive power, at least one surface of each lens is aspherical, the material of each lens is resin, the refractive index and Abbe number of the material of the first lens N1 and ν1, respectively, and the Abbe number of the material of the second lens is ν2,
N1 <1.54
ν1> ν2
A photographic lens characterized by satisfying the following conditions. Bf represents an air conversion distance from the image side surface of the fourth lens to the image plane, TL represents an air conversion distance from the aperture stop to the image plane, and a distance from the aperture stop to the image side surface of the fourth lens. Is ML and the focal length of the entire lens system is f,
bf / TL> 0.2
0.8 <ML / f <1.2
The photographing lens according to claim 1, wherein the following condition is satisfied. When the focal lengths of the first and second lenses are f1 and f2, respectively, and the focal length of the entire lens system is f,
0.80 <f1 / f <1.00
0.60 <| f2 | / f <0.80
The photographing lens according to claim 1, wherein the following condition is satisfied. When the radius of curvature of the lens surface on the object side of the second lens is r3, the radius of curvature of the lens surface on the object side of the third lens is r5, and the focal length of the entire lens system is f,
0.30 <| r3 | / f <0.50
0.80 <r5 / f <1.20
The photographic lens according to claim 1, 2 or 3, wherein the following condition is satisfied. When the distance from the aperture stop to the object side surface of the first lens is D0 and the focal length of the entire lens system is f,
D0 / f> 0.10
The photographic lens according to claim 1, wherein the following condition is satisfied.   6. The photographing lens according to claim 1, wherein the photographing lens is an optical system for forming an image on a solid-state image sensor.   An imaging apparatus comprising: the imaging lens according to claim 1; and a solid-state imaging device for photoelectrically converting an image formed by the imaging lens.