JP2006301222A - Super-wide angle lens - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a compact, thin-shaped super-wide angle lens of high optical performance, suitable for an on-vehicle television camera and a monitoring television camera, etc. <P>SOLUTION: A 1st biconcave lens 1 having negative refractive index, a 2nd lens 2 having negative refractive index, wherein the curvature radius of the surface on the object side is larger than that on the image field side, a 3rd lens 3 having positive refractive index, an aperture diaphragm SD of a prescribed diameter, and a 4th biconvex lens 4 having positive refractive index are arranged in order from the object side to the image field side. Regarding the super-wide angle lens, by introducing 4-group and 4-lens constitution, and also, making the 1st lens a negative lens, an appropriately wide diagonal viewing angle of ≥130° is secured, the whole lens length is shortened, and a back focus required to arrange an infrared cut filter or a low-pass filter required for a lens system for a digital camera equipped with a CCD or the like is secured. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an ultra-compact wide-angle lens used for a digital still camera or the like using a solid-state imaging device such as a CCD, and more particularly to a diagonal image applied to a lens system such as an in-vehicle TV camera or a surveillance TV camera. The present invention relates to a super wide angle lens having a wide angle of 130 degrees or more.

In recent years, with the widespread use of digital still cameras, there is an increasing demand for high performance, low cost, and compactness regarding lenses used in electronic photographing apparatuses. For example, many lens configurations that satisfy a wide angle, high performance, low cost, and compactness adopt a retrofocus type in which an emission angle is small while sufficiently ensuring a back focus.
In general, lenses used in surveillance TV cameras and in-vehicle TV cameras are required to be lightweight, compact and have a large angle of view, and cover a wide field of view, especially in applications such as in-vehicle TV cameras. Therefore, an ultra-wide-angle lens having a large angle of view of 130 ° or more is desired.
However, most of the conventional super wide-angle lenses of this type are generally composed of eight or ten or more lenses.

On the other hand, as a lens having a small number of lenses, a first meniscus lens having a convex surface facing the object side and a meniscus shape having a convex surface facing the object side are arranged in order from the object side to the image surface side. A super-wide-angle lens having a three-piece structure including a second lens and a biconvex third lens is known (for example, see Patent Document 1).
However, in this super-wide-angle lens, the outer diameter of the first lens is too large to meet the demand for miniaturization and compactness.

In addition, as a lens having a small number of lenses, a first meniscus lens having a convex surface facing the object side and a meniscus shape having a convex surface facing the object side, which are arranged in order from the object side to the image surface side. A four-lens super wide-angle lens composed of a second lens, a biconvex third lens, and a biconvex fourth lens is known (for example, see Patent Document 2).
However, this ultra-wide-angle lens does not meet the demand for cost reduction because glass lenses are used for all lenses.

JP 2003-195161 A JP-A-4-238312

  The present invention has been made in view of the above circumstances. The object of the present invention is a configuration of 4 elements in 4 groups, a diagonal angle of view of 130 degrees or more, and various aberrations, particularly distortion aberrations, are well corrected. Another object of the present invention is to provide a high-performance, compact, and inexpensive ultra-wide-angle lens compatible with an image sensor with about 300,000 pixels.

The super wide-angle lens of the present invention includes a first lens having a negative refractive index and a biconcave shape arranged in order from the object side to the image plane side, and having a negative refractive index than the image plane side. A second lens having a large curvature radius on the object side surface, a third lens having a positive refractive index, an aperture stop having a predetermined aperture, and a biconvex fourth lens having a positive refractive index; It is characterized by consisting of.
According to this configuration, by adopting a four-group four-lens configuration and making the first lens a negative lens having a negative refractive power, an appropriate wide angle of view with a diagonal angle of 130 ° or more can be secured, The total lens length can be shortened. Further, by using the retrofocus type, it is possible to secure a back focus for arranging an infrared light cut filter or a low-pass filter necessary for a lens system of a digital camera equipped with an image pickup device such as a CCD.

In the above configuration, the first lens, the second lens, and the fourth lens are formed such that both the object side and the image surface side are aspherical surfaces, and the refractive power becomes weaker toward the periphery. Can be adopted.
According to this configuration, since both surfaces of the first lens and the second lens are aspherical surfaces, particularly distortion can be corrected well, and a wide angle of view can be achieved.
Further, by making both surfaces of the lens (fourth lens) closest to the image surface aspherical, various aberrations can be corrected well while mainly correcting the coma aberration on the upper ray side.
For example, in order to obtain the same effect by making both surfaces of the lens (fourth lens) closest to the image surface spherical, it is necessary to have about two more lenses, and thinning cannot be achieved. By making both surfaces of the lens (fourth lens) closest to the surface aspherical, it is possible to satisfactorily correct various aberrations such as spherical aberration, astigmatism, and coma while achieving thinning. Optical performance can be obtained.

In the above configuration, a configuration in which at least one of the first lens, the second lens, the third lens, and the fourth lens is formed of a resin material can be employed.
According to this configuration, since at least one lens is a plastic lens formed of a resin material, the production cost can be reduced and the weight can be reduced as compared with the case where a glass lens is used. Since the spherical surface can be easily formed, it is possible to reduce the size by increasing the degree of freedom of aberration correction.

In the above configuration, when the focal length of the entire lens system is f and the distance from the image plane side surface of the fourth lens to the image plane is D9, the following conditional expression (1)
(1) 1.2 <D9 / f <1.4
A configuration that satisfies the above can be adopted.
According to this configuration, by satisfying conditional expression (1), it is possible to obtain a sufficient shooting angle of view while ensuring a space (back focus) necessary for arranging an infrared light cut filter or a low-pass filter. Thus, various aberrations, particularly distortion can be corrected well.

In the above configuration, when the focal length of the entire lens system is f, the combined focal length of the first lens and the second lens is f ₁₂ , and the focal length of the fourth lens is f ₄ , the following conditional expressions (2), ( 3)
_{(2) -2.9f <f 12 <} -2.5f
(3) 1.45f <f ₄ <1.95f
A configuration that satisfies the above can be adopted.
According to this configuration, by satisfying the conditional expressions (2) and (3), an appropriate back focus can be obtained while ensuring a sufficient shooting angle of view, and the outside of the first lens (front lens) can be obtained. Since the diameter can be reduced, downsizing and downsizing can be achieved.

In the above configuration, when the Abbe number of the first lens is ν1 and the Abbe number of the third lens is ν3, the following conditional expression (4)
(4) | ν1-ν3 |> 35
A configuration that satisfies the above can be adopted.
According to this configuration, by satisfying conditional expression (4), it is possible to satisfactorily correct chromatic aberration that affects resolution, particularly axial chromatic aberration and lateral chromatic aberration.

  According to the super wide-angle lens having the above-described configuration, the four-group four-lens configuration has a diagonal angle of view of 130 degrees or more, various aberrations, particularly distortion aberration, are well corrected, and is compatible with an imaging device having about 300,000 pixels. A super-wide-angle lens with high performance, compactness, and low cost can be obtained.

DESCRIPTION OF EXEMPLARY EMBODIMENTS Hereinafter, exemplary embodiments of the invention will be described with reference to the accompanying drawings.
1 and 2 show an embodiment of an ultra-wide-angle lens according to the present invention. FIG. 1 is a schematic configuration diagram, and FIG. 2 is an optical path diagram.
As shown in FIG. 1, this super-wide-angle lens has a negative refractive index, a biconcave first lens 1 arranged in order from the object side to the image plane side, and a negative refractive index. The second lens 2 having a radius of curvature larger on the object side surface than the image surface side, the third lens 3 having a positive refractive index, an aperture stop SD having a predetermined aperture, and a biconvex shape having a positive refractive index. It is formed by the fourth lens 4. An image plane P is disposed behind the fourth lens 4.

Here, the first lens 1, the second lens 2, the third lens 3, the aperture stop SD, the fourth lens 4, and the image plane P are sequentially arranged along the optical axis L from the object side to the image plane side. In this configuration, as shown in FIG. 1, each surface is Si (i = 1 to 9), the radius of curvature of each surface Si is Ri (i = 1 to 9), and the refractive index with respect to the d-line is Ni ( i = 1 to 4), Abbe number is νi (i = 1 to 4), and the distance (thickness, air distance) on each optical axis L from the first lens 1 to the image plane P is Di (i = 1). ~ 9).
The focal length of the entire lens system is f, the combined focal length of the first lens 1 and the second lens 2 is f ₁₂ , the focal length of the fourth lens 4 is f ₄ , and the image plane side surface of the fourth lens 4 The distance from the image surface P to the image plane P (back focus) is represented by D9.

The first lens 1 is preferably a resin material and is a biconcave lens in which the object-side surface S1 is concave and the image-side surface S2 is concave so as to have negative refractive power.
The second lens 2 is preferably a resin material, and is a meniscus lens in which the object-side surface S3 is convex and the image-side surface S4 is concave so as to have negative refractive power, The curvature radius R3 of the object side surface S3 is larger than the curvature radius R4 of the image side surface S4.
The third lens 3 is preferably a resin material and is a meniscus lens in which the object-side surface S5 is convex and the image-side surface S6 is concave so as to have a positive refractive index.
The fourth lens 4 is preferably a bi-convex lens formed of a resin material and having a positive refractive power and an object-side surface S8 having a convex shape and an image-side surface S9 having a convex shape. is there.

Thus, by adopting a four-group four-lens configuration including the first lens 1 to the fourth lens 4 and making the first lens 1 a negative lens having negative refractive power, the diagonal angle of view is 130 ° or more. An appropriate wide angle of view can be obtained, and the total lens length can be shortened.
In addition, by adopting the retrofocus type, it is possible to secure a space (back focus) for arranging an infrared light cut filter or a low-pass filter necessary for a lens system of a digital camera equipped with an image sensor such as a CCD. it can.
In addition, since at least one of the first lens 1 to the fourth lens 4 is formed of a resin material, the production cost can be reduced and the weight can be reduced as compared with the case where a glass lens is employed. In addition, since an aspherical surface can be easily formed, it is possible to reduce the size by increasing the degree of freedom of aberration correction.

Here, the fourth lens 4 is preferably formed such that both the object side and the image surface side S8, S9 are aspherical surfaces and the refractive power becomes weaker toward the periphery. As a result, various aberrations can be corrected well while mainly correcting the coma aberration on the upper ray side.
If both surfaces S8 and S9 of the lens closest to the image plane (fourth lens 4) are made spherical, about two more lenses are required to achieve the same effect, thereby achieving a reduction in thickness. It becomes difficult. Therefore, by making both surfaces S8 and S9 of the lens (fourth lens 4) closest to the image plane P aspherical, various aberrations such as spherical aberration, astigmatism, and coma are excellent while achieving thinning. Thus, high optical performance can be obtained.

  Further, the first lens 1 and the second lens 2 are preferably formed such that both the object side and the image surface side S1, S2, S3, and S4 are aspherical surfaces, and the refractive power becomes weaker toward the periphery. Is done. Thereby, especially a distortion aberration can be correct | amended favorably and a wide view angle can be achieved.

Here, the expression representing the aspheric surfaces of the first lens 1 to the fourth lens 4 is defined by the following expression.
Z = Cy ² / [1+ (1-εC ² y ² ) ^1/2 ] + Dy ⁴ + Ey ⁶ + Fy ⁸ + Gy ¹⁰
Where Z: distance from the tangent plane at the apex of the aspheric surface to a point on the aspheric surface whose height from the optical axis L is y, y: height from the optical axis, C: curvature at the apex of the aspheric surface ( 1 / R), ε: conic constant, D, E, F, G: aspheric coefficient.

In the above configuration, the focal length f of the entire lens system and the distance D9 from the image plane side surface S9 to the image plane P of the fourth lens 4 are preferably the following conditional expression (1):
(1) 1.2 <D9 / f <1.4
It is formed so as to satisfy.
Conditional expression (1) defines the back focus (distance D9) required for the CCD camera lens. When the value of D9 / f exceeds the upper limit value and the back focus becomes longer, the generated negative distortion becomes larger. On the other hand, when the value of D9 / f is less than the lower limit and the back focus is shortened, it becomes difficult to sufficiently increase the obtained field angle of view, and an infrared light cut filter or a low-pass filter is disposed. It becomes difficult to secure space.
Therefore, by satisfying conditional expression (1), it is possible to obtain a sufficient shooting angle of view while ensuring a space (back focus) necessary for arranging an infrared light cut filter or a low-pass filter. Aberrations, particularly distortion, can be corrected well.

In the above configuration, the focal length f of the entire lens system, the combined focal length f _{12 of} the first lens 1 and the second lens 2, and the focal length of the fourth lens 4 are f ₄ , preferably the following conditional expression (2), (3)
_{(2) -2.9f <f 12 <} -2.5f
(3) 1.45f <f ₄ <1.95f
It is formed so as to satisfy.
By satisfying conditional expressions (2) and (3), it is possible to obtain an appropriate back focus while securing a sufficient shooting angle of view. Moreover, since the outer diameter of the 1st lens 1 (front lens) can be made small, size reduction and compactization can be achieved. If a focal length equivalent to the conditional expression (2) is obtained with only the first lens in the three-group three-lens configuration, the curvature radius on the image plane side of the first lens becomes very small, and the lens is processed. Becomes difficult.

Furthermore, in the above-described configuration, the Abbe number ν1 of the first lens 1 and the Abbe number ν3 of the third lens 3 are preferably the following conditional expression (4):
(4) | ν1-ν3 |> 35
It is formed so as to satisfy.
By satisfying conditional expression (4), it is possible to satisfactorily correct chromatic aberration that affects resolution, particularly axial chromatic aberration and lateral chromatic aberration.

  Next, specific numerical examples of the super-wide-angle lens will be described below as Example 1 and Example 2.

Numerical data of conditional expressions (1) to (4) in the first embodiment, main specifications of the first lens 1 to the fourth lens 4, and various numerical data (setting values) are as follows.
<Value of conditional expression>
(1) D9 / f = 1.33 / 1.04 = 1.28
_{(2) f = 1.04, f} 12 = -2.75, -3.02 <-2.75 <-2.60
(3) f = 1.04, f ₄ = 1.64, 1.51 <1.64 <2.03
(4) ν1 = 56.3, ν3 = 18.9, | ν1−ν3 | = | 56.3-18.9 | = 37.4> 35

<Specification specifications>
Object distance = infinity, focal length of entire lens system (f) = 1.04 mm, combined focal length of first lens and second lens (f ₁₂ ) = − 2.75 mm, focal length of fourth lens (f ₄ ) = 1.64 mm, F-number = 2.8, exit pupil position = 2.38 mm, total lens length = 13.0 mm, back focus (air conversion) = 1.33 mm, field angle (2ω) = 140.7 °, Distortion (maximum value) = -23.8%
<Aspherical surface>
Both surfaces S1, S2 of the first lens 1, both surfaces S3, S4 of the second lens 2, and both surfaces S8, S9 of the fourth lens 4
<Curvature radius>
R1 = −43.304 mm (aspheric surface), R2 = 3.134 mm (aspheric surface), R3 = 8.762 mm (aspheric surface), R4 = 2.745 mm (aspheric surface), R5 = 3.542 mm, R6 = 16 .083 mm, R7 = ∞, R8 = 4.567 mm (aspheric surface), R9 = −0.838 mm (aspheric surface)
<Spacing on the optical axis>
D1 = 1.830 mm, D2 = 1.451 mm, D3 = 1.005 mm, D4 = 0.478 mm, D5 = 2.33 mm, D6 = 0.498 mm, D7 = 0.896 mm, D8 = 2.743 mm, D9 = 1.333mm
<Refractive index (Nd)>
N1 = 1.525120, N2 = 1.525120, N3 = 1.922860, N4 = 1.525120
<Abbe number (νd)>
ν1 = 56.3, ν2 = 56.3, ν3 = 18.9, ν4 = 56.3

<Numerical data of aspheric coefficient>
<S1 surface>
ε = −4.919041, D = 2.521963 × 10 ⁻⁵ , E = 8.287960 × 10 ⁻⁷ , F = 1.271981 × 10 ⁻⁸ , G = −7.667472 × 10 ⁻¹⁰
<S2 surface>
ε = 0.069026, D = 0.016926, E = -1.78512 × 10 ⁻³ , F = −2.881600 × 10 ⁻⁵ , G = −1.0032020 × 10 ⁻⁵
<S3 surface>
ε = 8.575587, D = 1.616654 × 10 ⁻⁴ , E = 3.665278 × 10 ⁻⁵ , F = −2.39254 × 10 ⁻⁵ , G = −8.001936 × 10 ⁻⁴
<S4 surface>
ε = −1.327210, D = −0.036409, E = 0.013830, F = −2.38200 × 10 ⁻³ , G = 2.130445 × 10 ⁻⁴
<S8 surface>
ε = 0.250456, D = −0.039621, E = 0.0189291, F = −4.779312 × 10 ⁻³ , G = 5.645256 × 10 ⁻⁴
<S9 surface>
ε = -1.198487, D = 0.0654498, E = −0.024298, F = 4.330720 × 10 ⁻³ , G = −1.83991 × 10 ⁻⁴

The aberration diagram regarding the spherical aberration, astigmatism, distortion (distortion) and lateral chromatic aberration in Example 1 is as shown in FIG. In FIG. 3, S represents the aberration on the sagittal plane, and M represents the aberration on the meridional plane.
According to the lens specifications according to Example 1, the total lens length is 13.0 mm, the F number is 2.8, the angle of view (2ω) is 140.7 °, and particularly, distortion is corrected well, and the optical performance is high. A thin and small super wide-angle lens can be obtained.

Numerical data of the conditional expressions (1) to (4) in the second embodiment, main specification specifications of the first lens 4 to the fourth lens 4, and various numerical data (setting values) are as follows.
<Value of conditional expression>
(1) D9 / f = 1.33 / 1.07 = 1.24
_{(2) f = 1.07, f} 12 = -2.78, -3.07 <-2.78 <-2.65
(3) f = 1.07, f ₄ = 1.64, 1.54 <1.64 <2.07
(4) ν1 = 64.1, ν3 = 18.9, | ν1-ν3 | = | 64.1-18.9 | = 45.2> 35

<Specification specifications>
Object distance = infinity, focal length of entire lens system (f) = 1.07 mm, combined focal length of first lens and second lens (f ₁₂ ) = − 2.78 mm, focal length of fourth lens (f ₄ ) = 1.64 mm, F-number = 2.8, exit pupil position = 2.54 mm, total lens length = 12.7 mm, back focus (air conversion) = 1.33 mm, field angle (2ω) = 142.1 °, Distortion (maximum value) = -28.7%
<Aspherical surface>
Both surfaces S1, S2 of the first lens 1, both surfaces S3, S4 of the second lens 2, and both surfaces S8, S9 of the fourth lens 4
<Curvature radius>
R1 = −42.889mm (aspherical surface), R2 = 3.134 mm (aspherical surface), R3 = 8.753 mm (aspherical surface), R4 = 2.752 mm (aspherical surface), R5 = 3.517 mm, R6 = 16 .051 mm, R7 = ∞, R8 = 4.413 mm (aspheric surface), R9 = −0.842 mm (aspheric surface)
<Spacing on the optical axis>
D1 = 1.705 mm, D2 = 1.445 mm, D3 = 1.029 mm, D4 = 0.451 mm, D5 = 2.322 mm, D6 = 0.456 mm, D7 = 0.659 mm, D8 = 2.713 mm, D9 = 1.333mm
<Refractive index (Nd)>
N1 = 1.516330, N2 = 1.525120, N3 = 1.922860, N4 = 1.525120
<Abbe number (νd)>
ν1 = 64.1, ν2 = 56.3, ν3 = 18.9, ν4 = 56.3

<Numerical data of aspheric coefficient>
<S1 surface>
ε = −3.088380, D = 2.42501 × 10 ⁻⁵ , E = 8.7068 × 10 ⁻⁷ , F = 1.439649 × 10 ⁻⁸ , G = −7.20092 × 10 ⁻¹⁰
<S2 surface>
ε = 0.068948, D = 0.016966, E = −1.78133 × 10 ⁻³ , F = 3.448596 × 10 ⁻⁵ , G = −2.381616 × 10 ⁻⁵
<S3 surface>
ε = 8.597569, D = 1.124966 × 10 ⁻⁴ , E = 3.448596 × 10 ⁻⁵ , F = −2.38019 × 10 ⁻⁵ , G = −7.9818 × 10 ⁻⁶
<S4 surface>
ε = −1.273547, D = −0.036151, E = 0.013847, F = −2.38603 × 10 ⁻³ , G = 2.114568 × 10 ⁻⁴
<S8 surface>
ε = 0.246821, D = −0.039653, E = 0.0138351, F = −4.778373 × 10 ⁻³ , G = 5.625891 × 10 ⁻⁴
<S9 surface>
ε = −1.201276, D = 0.065733, E = −0.024271, F = 4.307228 × 10 ⁻³ , G = −1.87138 × 10 ⁻⁴

The aberration diagram regarding the spherical aberration, astigmatism, distortion (distortion) and lateral chromatic aberration in Example 2 is as shown in FIG. In FIG. 4, S represents the aberration on the sagittal plane, and M represents the aberration on the meridional plane.
According to the lens specifications according to the second embodiment, the total lens length is 12.7 mm, the F number is 2.8, the field angle (2ω) is 142.1 °, and particularly, distortion is corrected well and the optical performance is high. A thin and small super wide-angle lens can be obtained.

  As described above, the ultra-wide-angle lens of the present invention is ultra-compact and thin and can secure high optical performance that can accommodate an image sensor with about 300,000 pixels. Of course, it can be applied as a wide-angle lens, and is also useful as a super-wide-angle lens of a camera used for other purposes.

It is a schematic block diagram which shows one Embodiment of the super wide angle lens which concerns on this invention. FIG. 2 is an optical path diagram of the super wide angle lens shown in FIG. 1. FIG. 3 is an aberration diagram showing spherical aberration, astigmatism, distortion, and lateral chromatic aberration in Example 1. FIG. 6 is an aberration diagram showing spherical aberration, astigmatism, distortion, and lateral chromatic aberration in Example 2.

Explanation of symbols

L Optical axis f Focal length f of entire lens system ₁₂ Synthetic focal length f of first lens and second lens f ₄ Focal length of fourth lens 1 First lens 2 Second lens 3 Third lens 4 Fourth lens SD Aperture stop P Image plane ν1 Abbe number of the first lens ν3 Abbe number of the third lens D9 Distance on the optical axis from the image plane side surface of the fourth lens to the image plane

Claims (6)

Arranged in order from the object side to the image plane side,
A biconcave first lens having a negative refractive index;
A second lens having a negative refractive index and a larger radius of curvature of the object side surface than the image surface side;
A third lens having a positive refractive index;
An aperture stop having a predetermined aperture;
A biconvex fourth lens having a positive refractive index;
An ultra-wide-angle lens characterized by comprising The first lens, the second lens, and the fourth lens are formed such that both the object side and the image side are aspherical surfaces, and the refractive power becomes weaker toward the periphery.
The super-wide-angle lens according to claim 1. At least one of the first lens, the second lens, the third lens, and the fourth lens is formed of a resin material.
The super-wide-angle lens according to claim 1 or 2, The following conditional expression (1) is satisfied, where f is the focal length of the entire lens system, and D9 is the distance from the image plane side surface to the image plane of the fourth lens. The super wide-angle lens according to any one of the above.
(1) 1.2 <D9 / f <1.4 When the focal length of the entire lens system is f, the combined focal length of the first lens and the second lens is f ₁₂ , and the focal length of the fourth lens is f ₄ , the following conditional expressions (2) and (3) are satisfied. The super-wide-angle lens according to claim 1, wherein the super-wide-angle lens is satisfied.
_{(2) -2.9f <f 12 <} -2.5f
(3) 1.45f <f ₄ <1.95f The super condition according to claim 1, wherein when the Abbe number of the first lens is ν1 and the Abbe number of the third lens is ν3, the following conditional expression (4) is satisfied. Wide angle lens.
(4) | ν1-ν3 |> 35

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