JP2007310195A - Imaging optical system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an imaging optical system which is compact and has satisfactorily corrected aberration. <P>SOLUTION: The imaging optical system has a first lens which has a convex surface facing an imaging object side and has positive refractive power, a second lens which has a convex surface facing the image surface side and has positive refractive power and a third lens which has a meniscus shape having a convex surface facing the image surface side and has negative refractive power, and the imaging optical system satisfies the following condition expression, 3.6<f23/f<50, wherein f23 is a composite focal distance of the second lens and the third lens and f is a focal distance of the whole system. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an image pickup optical system capable of guiding light to an image pickup device, and more particularly to an image pickup optical system suitable for a digital camera or a mobile phone.

  In recent years, with the spread of personal computers, digital cameras that can easily capture images have become widespread. In addition, it has become common to incorporate a digital camera into an information processing device such as a mobile computer, a mobile phone, or a personal digital assistant (PDA). Under such circumstances, a smaller digital camera has been demanded, and further downsizing of the imaging optical system has been demanded.

  As a method for reducing the size of the imaging optical system, there is a method for reducing the size of the imaging element. In this method, it is necessary to reduce the size of each light receiving portion of the image sensor. For this reason, the imaging optical system must focus the light beam on a small light receiving section, and a high-performance imaging optical system is required.

  On the other hand, there is a method of reducing the size of the imaging optical system while keeping the size of the imaging element as it is. This method can shorten the overall length of the imaging optical system, but the exit pupil position of the imaging optical system also approaches the image plane. When the exit pupil position approaches the image plane, the off-axis light beam emitted from the imaging optical system enters the image plane obliquely. Then, a greatly inclined light beam enters the microlens provided on the front surface of the light receiving unit of the image sensor, and the amount of light at the peripheral part of the image sensor is reduced. As a result, there arises a problem that the amount of light on the image plane is extremely different between the central portion and the peripheral portion of the image plane. There is a demand for a high-performance imaging optical system that is compact and has a uniform amount of light on the image plane.

In view of the above problems and circumstances, Patent Document 1 proposes an imaging optical system having a compact three-lens configuration.
JP 2004-37960 A

  However, although Patent Document 1 has a compact configuration, there is a problem that the exit pupil position is close to the image plane with respect to the entire length of the imaging optical system, and there is a difference in the amount of light between the central portion and the peripheral portion of the image plane. is doing.

  The present invention has been made to solve the above-described problems, and an object of the present invention is to provide an imaging optical system in which aberrations are favorably corrected while being small in size.

The above problem is solved by the following configuration.
1.
In an imaging optical system having a plurality of lenses and imaging light from an imaging target on an imaging device,
In order from the imaging target, a first lens, an aperture stop, a second lens, and a third lens are provided,
The first lens has a convex surface on the imaging target side and has a positive refractive power,
The second lens has a positive refractive power with a convex surface facing the image surface side,
The third lens has a meniscus shape with a convex surface facing the image surface side and has negative refractive power,
An imaging optical system characterized by satisfying the following conditional expression:

3.6 <f23 / f <50
However,
f23: Composite focal length of the second lens and the third lens f: Focal length of the entire system
2. The imaging optical system according to 1, wherein the first lens has a meniscus shape with a convex surface facing the imaging target side.
3.
3. The imaging optical system according to 1 or 2, wherein the second lens has a meniscus shape with a convex surface facing the image surface side.
4).
The imaging optical system according to any one of claims 1 to 3, wherein each of the first lens, the second lens, and the third lens has at least one aspheric surface.
5).
The imaging optical system according to any one of 1 to 4, wherein the following conditional expression is satisfied.

−5 <(r31 + r32) / (r31−r32) <− 1
However,
r31: radius of curvature of the third lens on the imaging target side r32: radius of curvature of the third lens on the image plane side

  In the present invention, a first positive refractive power lens, an aperture stop, a second positive refractive power lens, and a third negative refractive power lens are arranged in order from the imaging target side, and the second positive refractive power lens and the third positive refractive power lens are arranged. By setting the composite focal length of the negative refracting power lens to an appropriate range, the entire length of the imaging optical system can be shortened, and deterioration of the optical performance can be suppressed.

  Hereinafter, preferred embodiments of the present invention will be described with reference to the drawings.

  1 to 3 are cross-sectional views showing lens arrangements of first to third embodiments of the imaging optical system CL according to the present invention.

  1 to 3, the imaging optical system CL includes, in order from the imaging target side, a first lens FL1 of a positive meniscus lens having a convex surface facing the imaging target side, an aperture stop A, and a positive meniscus having a convex surface facing the image plane side. The lens includes a second lens FL2 of the lens, a third lens FL3 of a negative meniscus lens having a convex surface facing the image surface side, and a glass filter GF.

  Next, conditional expressions to be satisfied by the photographing optical system CL of each embodiment will be described. The imaging optical system CL of each embodiment does not have to satisfy all of the following conditional expressions at the same time, and can achieve corresponding effects by satisfying each conditional expression independently. It is. Needless to say, satisfying a plurality of conditional expressions is more desirable from the viewpoint of optical performance, miniaturization, or assembly.

3.6 <f23 / f <50 (1)
Here, f23 is a combined focal length of the second lens and the third lens, and f is a focal length of the entire system.

  If the upper limit of conditional expression (1) is exceeded, coma will deteriorate, Petzval sum will be overcorrected, and resolution will be insufficient. If the lower limit of conditional expression (1) is exceeded, the Petzval sum becomes insufficiently corrected, making it difficult to correct field curvature.

  It is further desirable to satisfy the following conditional expressions (1A) and (1B).

4 <f23 / f <30 (1A)
5 <f23 / f <25 (1B)
−5 <(r31 + r32) / (r31−r32) <− 1 (2)
Here, r31: radius of curvature of the third lens on the imaging target side, r32: radius of curvature of the third lens on the image plane side.

  If the upper limit of conditional expression (2) is exceeded, coma will deteriorate, Petzval sum will be overcorrected, and resolution will be insufficient. If the lower limit of conditional expression (2) is exceeded, the Petzval sum becomes insufficiently corrected, making it difficult to correct field curvature.

  It is further desirable to satisfy the following conditional expressions (2A) and (2B).

−2 <(r31 + r32) / (r31−r32) <− 1.06 (2A)
-1.5 <(r31 + r32) / (r31-r32) <-1.1 (2B)
-1.52 <(r11 + r12) / (r11-r12) <-1 (3)
Here, r11: the radius of curvature of the first lens on the imaging target side, r12: the radius of curvature of the first lens on the image plane side.

  If the upper limit of conditional expression (3) is exceeded, the Petzval sum becomes insufficiently corrected, making it difficult to correct field curvature. If the lower limit of conditional expression (3) is exceeded, coma will deteriorate, Petzval sum will be overcorrected, and resolution will be insufficient.

  It is further desirable to satisfy the following conditional expressions (3A) and (3B).

-1.51 <(r11 + r12) / (r11-r12) <-1 (3A)
−1.5 <(r11 + r12) / (r11−r12) <− 1 (3B)
It is desirable that each of the first lens, the second lens, and the third lens of the imaging optical system according to the present invention has at least one aspheric surface. This has a great effect on correction of spherical aberration, coma aberration, and distortion aberration.

  Specific examples of the present invention will be described below with reference to construction data, aberration diagrams, and the like.

  Examples 1 to 3 listed below correspond to the first to third embodiments described above, respectively, and the lens arrangement diagrams representing the embodiments show the lens configurations of the corresponding Examples 1 to 3, respectively. ing.

  In each embodiment, ri (i = 1, 2, 3,...) Is the radius of curvature of the i-th surface counted from the imaging target side, and di (i = 1, 2, 3,...) Is the imaging target side. I (i = 1, 2, 3,...) And νi (i = 1, 2, 3,...) Are the i-th lens counts from the imaging target side. The refractive index and Abbe number for the d-line are shown. F represents the focal length of the entire system, and FNO represents the F number.

  Further, in each of the examples, the surface marked with * in the radius of curvature ri indicates an aspherical refractive optical surface or a surface having a refractive action equivalent to an aspherical surface, and represents the surface shape of the aspherical surface. It shall be defined by the formula of

X (H) = C · H ² / {1 + √ (1−ε · C ² · H ² )} + ΣAi · Hi (AS)
Here, H is the height in the direction perpendicular to the optical axis, X (H) is the amount of displacement in the optical axis direction at the position of the height H (based on the surface vertex), C is the paraxial curvature, and ε is 2 The secondary curved surface parameter, Ai is the i-th order aspheric coefficient, and Hi is H to the power of i.
Example 1
The configuration of the imaging optical system CL is shown in FIG. f = 4.37 mm and FNO = 4.0. The construction data of the imaging optical system CL is shown in Table 1.

  The aspheric surface coefficients are shown below.

Aspherical coefficient of the first surface (r1) ε = 0.85518 × 10
A4 = −0.61806 × 10 ⁻²
A6 = −0.83936 × 10 ⁻²
A8 = 0.12016 × 10 ⁻¹
A10 = −0.86799 × 10 ⁻²
A12 = 0.17054 × 10 ⁻²
Aspheric coefficient of the second surface (r2) ε = −0.90000 × 10
A4 = 0.14467 × 10 ⁻³
A6 = −0.40101 × 10 ⁻¹
A8 = 0.13473
A10 = −0.17543
Aspherical coefficient of the fourth surface (r4) ε = −0.14116
A4 = −0.71999 × 10 ⁻¹
A6 = 0.24417 × 10 ⁻²
A8 = 0.10224
A10 = −0.44325 × 10 ⁻¹
Aspherical coefficient of the fifth surface (r5) ε = −0.17379 × 10
A4 = −0.15069
A6 = 0.82394 × 10 ⁻¹
A8 = −0.17004 × 10 ⁻¹
A10 = 0.35881 × 10 ⁻²
A12 = 0.606133 × 10 ⁻³
Aspherical coefficient of the sixth surface (r6) ε = −0.40000 × 10
A4 = 0.47924 × 10 ⁻¹
A6 = −0.58460 × 10 ⁻²
A8 = −0.29127 × 10 ⁻²
A10 = 0.75237 × 10 ⁻³
A12 = −0.475556 × 10 ⁻⁴
Aspherical coefficient of the seventh surface (r7) ε = 0.8000 × 10
A4 = 0.188840 × 10 ⁻¹
A6 = −0.49565 × 10 ⁻²
A8 = 0.16100 × 10 ⁻³
A10 = −0.10249 × 10 ⁻⁴
A10 = 0.115536 × 10 ⁻⁵
(Example 2)
The configuration of the imaging optical system CL is shown in FIG. f = 4.51 mm and FNO = 2.9. Table 2 shows construction data of the imaging optical system CL.

  The aspheric surface coefficients are shown below.

Aspherical coefficient of the first surface (r1) ε = 0.17389 × 10
A4 = −0.96357 × 10 ⁻²
A6 = −0.84693 × 10 ⁻²
A8 = 0.863320 × 10 ⁻²
A10 = −0.36530 × 10 ⁻²
A12 = −0.21348 × 10 ⁻³
Aspheric coefficient of the second surface (r2) ε = −0.69944 × 10
A4 = −0.16766 × 10 ⁻²
A6 = −0.28183 × 10 ⁻¹
A8 = 0.895564 × 10 ⁻¹
A10 = −0.79855 × 10 ⁻¹
Aspherical coefficient of the fourth surface (r4) ε = 0.94304
A4 = −0.10224
A6 = −0.19790 × 10 ⁻¹
A8 = 0.10404
A10 = −0.18595 × 10 ⁻¹
Aspherical coefficient of the fifth surface (r5) ε = −0.13968 × 10
A4 = −0.17577
A6 = 0.71061 × 10 ⁻¹
A8 = −0.15956 × 10 ⁻¹
A10 = 0.87997 × 10 ⁻²
A12 = 0.55219 × 10 ⁻³
Aspherical coefficient of the sixth surface (r6) ε = −0.40000 × 10
A4 = 0.0.606889 × 10 ⁻¹
A6 = −0.73649 × 10 ⁻²
A8 = −0.32892 × 10 ⁻²
A10 = 0.696648 × 10 ⁻³
A12 = −0.23032 × 10 ⁻⁴
Aspherical coefficient of the seventh surface (r7) ε = 0.8000 × 10
A4 = 0.115548 × 10 ⁻¹
A6 = −0.21917 × 10 ⁻²
A8 = −0.53916 × 10 ⁻⁴
A10 = −0.10261 × 10 ⁻³
A12 = 0.13151 × 10 ⁻⁴
(Example 3)
The configuration of the imaging optical system CL is shown in FIG. f = 5.15 mm and FNO = 3.0. Table 3 shows construction data of the imaging optical system CL.

  The aspheric surface coefficients are shown below.

Aspheric coefficient of first surface (r1) ε = 0.19167 × 10
A4 = −0.31011 × 10 ⁻²
A6 = −0.52787 × 10 ⁻²
A8 = 0.532224 × 10 ⁻²
A10 = −0.22782 × 10 ⁻³
A12 = −0.61464 × 10 ⁻³
Aspheric coefficient of the second surface (r2) ε = −0.90000 × 10
A4 = 0.13730 × 10 ⁻¹
A6 = −0.12404 × 10 ⁻¹
A8 = 0.56050 × 10 ⁻¹
A10 = −0.41881 × 10 ⁻¹
Aspherical coefficient of the fourth surface (r4) ε = 0.14978 × 10
A4 = −0.64390 × 10 ⁻¹
A6 = −0.36198 × 10 ⁻¹
A8 = 0.487717 × 10 ⁻¹
A10 = −0.59089 × 10 ⁻²
Aspheric coefficient of the fifth surface (r5) ε = −0.13992 × 10
A4 = −0.11340
A6 = 0.243472 × 10 ⁻¹
A8 = −0.888880 × 10 ⁻²
A10 = 0.17284 × 10 ⁻²
A12 = 0.84375 × 10 ⁻³
Aspherical coefficient of the sixth surface (r6) ε = −0.30948 × 10
A4 = 0.38243 × 10 ⁻¹
A6 = −0.34987 × 10 ⁻²
A8 = −0.18222 × 10 ⁻²
A10 = 0.85528 × 10 ⁻³
A12 = 0.11897 × 10 ⁻⁴
Aspherical coefficient of the seventh surface (r7) ε = 0.26375 × 10
A4 = 0.12418 × 10 ⁻¹
A6 = −0.30271 × 10 ⁻²
A8 = 0.40981 × 10 ⁻⁴
A10 = −0.53462 × 10 ⁻⁶
A12 = 0.26078 × 10 ⁻⁶
Table 4 also shows values corresponding to the parameters defined by the conditional expressions (1), (2), and (3) of each example.

  4 to 6 are aberration diagrams corresponding to Examples 1 to 3. FIG. Each aberration diagram shows a spherical aberration diagram, an astigmatism diagram, and a distortion diagram in order from the left side.

  In each spherical aberration diagram, the solid line d represents the d line, the alternate long and short dash line g represents the g line, the alternate long and two short dashes line c represents the amount of spherical aberration for each of the c lines, and SC represents the unsatisfactory sine condition. In each astigmatism diagram, the solid line DS represents the sagittal plane, and the dotted line DM represents the meridional plane. Also, the vertical axis of the spherical aberration diagram represents the F number of the light beam, and the vertical axes of the astigmatism diagram and the distortion diagram represent the maximum image height Y ′.

  As described above, according to the imaging optical system of the present invention, it is possible to provide a compact imaging optical system for a solid-state imaging device with good optical performance.

  Therefore, when the imaging optical system according to the present invention is applied to an imaging optical system such as a digital camera, it can contribute to higher functionality and downsizing of the camera.

The lens block diagram of the imaging optical system of 1st Embodiment (Example 1). The lens block diagram of the imaging optical system of 2nd Embodiment (Example 2). FIG. 6 is a lens configuration diagram of an imaging optical system according to a third embodiment (Example 3). FIG. 6 is an aberration diagram of the image pickup optical system according to the first embodiment. FIG. 6 is an aberration diagram of the image pickup optical system according to the second embodiment. FIG. 10 is an aberration diagram of the image pickup optical system according to the third embodiment.

Explanation of symbols

CL imaging optical system FL1 1st lens FL2 2nd lens FL3 3rd lens A Aperture GF Glass filter r1-r9 surface

Claims (5)

In an imaging optical system having a plurality of lenses and imaging light from an imaging target on an imaging device,
In order from the imaging target, a first lens, an aperture stop, a second lens, and a third lens are provided,
The first lens has a convex surface on the imaging target side and has a positive refractive power,
The second lens has a positive refractive power with a convex surface facing the image surface side,
The third lens has a meniscus shape with a convex surface facing the image surface side and has negative refractive power,
An imaging optical system characterized by satisfying the following conditional expression:
3.6 <f23 / f <50
However,
f23: Composite focal length of the second lens and the third lens f: Focal length of the entire system The imaging optical system according to claim 1, wherein the first lens has a meniscus shape with a convex surface facing the imaging target side. The imaging optical system according to claim 1, wherein the second lens has a meniscus shape with a convex surface facing the image surface side. 4. The imaging optical system according to claim 1, wherein each of the first lens, the second lens, and the third lens has at least one aspheric surface. 5. The imaging optical system according to any one of claims 1 to 4, wherein the following conditional expression is satisfied.
−5 <(r31 + r32) / (r31−r32) <− 1
However,
r31: radius of curvature of the third lens on the imaging target side r32: radius of curvature of the third lens on the image plane side

Images