JP2005115309A - Imaging lens - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To propose a compact and inexpensive imaging lens constituted of three groups and coping with mega-pixel. <P>SOLUTION: The imaging lens 10 is constituted of three groups, and has the 1st lens group I being a meniscus turning its convex surface toward an object side and having negative power, a 2nd lens group II turning its concave surface toward the object side and having positive power and the 3rd lens group III having negative power which are arranged in order from the object side. At least one lens surface out of the lens surfaces of the 1st and the 2nd lens groups is aspherical, and both lens surfaces of the 3rd lens group are aspherical. Assuming that the composite focal length of the lens 10 is F, the focal length of the 1st lens is f1, the focal length of the 3rd lens is f3, and space on an optical axis from the lens surface on the object side of the 1st lens 1 to an image forming surface is Σd, (1) 0.5<Σd<F<3.0, (2) 0.5<¾f3/F¾<1.6 and (3) -10<f1/F<-5 are satisfied. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a small and lightweight imaging lens used for an in-vehicle camera, a monitoring camera, a digital camera, a mobile phone camera, and the like using a light receiving element such as a CCD or a CMOS.

  An imaging lens incorporated in a monitoring camera, a digital camera, or the like using a light receiving element such as a CCD or CMOS desirably has faithful subject reproducibility. Recently, the CCD itself and the CCD camera have been downsized, and along with this, the demand for downsizing and downsizing of the imaging lens incorporated therein has inevitably increased. Furthermore, light receiving elements such as CCDs have become higher in the order of mega pixels, contrary to the miniaturization of CCDs. An imaging lens used for a camera using this must inevitably have a high optical performance. Conventionally, aberrations are corrected using a large number of lenses in order to exhibit high optical performance.

  Further, as a feature of a light receiving element such as a CCD or a CMOS, there is a restriction on a light ray angle taken into each pixel. In an optical system that ignores this, the aperture efficiency decreases and shading occurs.

  In addition, since a space for inserting a low-pass filter, an infrared cut filter, or the like is required between the imaging lens and the CCD, there is a restriction that the back focus must be long to some extent.

Patent Document 1 below discloses an imaging lens having a three-group three-lens configuration that can satisfactorily correct the curvature of field of the three-group three-lens configuration, and that is small, light, and low-cost. However, according to the embodiment described here, the total length from the object-side lens surface of the first lens to the imaging surface is relatively long, and it is not necessarily said that the size is reduced.
JP-A-10-170819

  It is an object of the present invention to propose an imaging lens that is aberration-corrected so as to be compatible with mega-order high pixels, shading is suppressed, and that it is small, lightweight, compact, and inexpensive.

  The imaging lens of the present invention has, for example, a three-group three-lens configuration, and in order from the object side, a first lens group including a meniscus first lens having a negative power with a convex surface facing the object side, and an object side. A second lens group including a second lens having a positive power with a concave surface facing and a third lens group including a third lens having a negative power are arranged. Of the lens surfaces of the first and second lenses, at least one lens surface is aspheric, and the lens surfaces on both sides of the third lens are aspheric.

  By making the first lens negative, a stable wide-angle lens can be obtained, and at the same time, a predetermined back focus can be secured. Further, by making the curvature of the lens surface on the object side of the second lens negative, distortion and other aberration correction can be facilitated.

Next, in the imaging lens of the present invention, the combined focal length of the imaging lens is F, the focal length of the first lens is f1, the focal length of the third lens is f3, and an image is formed from the object-side lens surface of the first lens. When the interval on the optical axis to the surface is Σd, the following conditional expression is satisfied.
0.5 <Σd / F <3.0 (1)
0.5 <| f3 / F | <1.6 (2)
−10 <f1 / F <−0.5 (3)

  Conditional expression (1) is a condition for miniaturization. If the lower limit is not reached, the optical system becomes smaller and processing becomes difficult. If the upper limit is exceeded, the purpose of miniaturization and compactness is not satisfied. In particular, it is desirable that F <1.5.

  Conditional expression (2) is a condition for satisfactorily correcting chromatic aberration of magnification and a condition for maintaining good distortion. If the lower limit is not reached, the exit pupil is shortened and shading is caused, which is not preferable. If the upper limit is exceeded, the exit pupil becomes longer, but correction of chromatic aberration becomes difficult. In addition, by setting the upper limit of conditional expression (2) to a range of | f3 / F | <1.6, the maximum emission angle of the principal ray can be reduced.

  Conditional expression (3) is for stably widening the entire optical system and for ensuring sufficient back focus. If the lower limit is not reached, it is difficult to correct various aberrations for widening, and if the upper limit is exceeded, the back focus becomes longer and compactness cannot be achieved.

Next, in the imaging lens of the present invention, when the thickness of the first lens is d1,
0.1 <d1 / F <0.4 (4)
It is desirable that

  Conditional expression (4) is related to conditional expression (1) and is a condition for stabilizing the chromatic aberration of magnification, curvature of field and distortion, and minimizing the lens diameter. When the lower limit is 0.1 or less, the lens diameter can be reduced, but the edge thickness of the lens becomes extremely small, which deteriorates the workability of the lens and increases the cost. At the same time, it is difficult to secure the amount of light in the periphery. In addition, the curvature of the image plane cannot be kept stable. On the other hand, when the upper limit of 0.4 is exceeded, the radius of curvature of the lens surface decreases and the chromatic aberration of magnification increases, and the curvature and distortion of the imaging surface increase, making it difficult to obtain stable imaging characteristics. In addition, the lens system cannot be downsized. The range of the value of d1 / F is preferably 0.15 <d1 / F <0.3.

Next, in the imaging lens of the present invention, the air gap da constituted by the first lens and the second lens is set to da / F <0.3 (5)
By keeping it small so as to satisfy the above, it is possible to reduce the size of the entire optical system and increase the amount of peripheral light.

  Here, as a feature when the imaging surface is a CCD or a CMOS, there is a restriction on a light ray angle taken into each pixel, and the light ray angle increases toward the periphery of the screen. In order to alleviate this phenomenon, it is desirable to make the maximum emission angle of the chief ray 30 degrees or less by utilizing an aspherical shape.

  On the other hand, the imaging lens of the present invention can also be configured to have four elements in three groups. In this case, the second lens constituting the second lens group is a synthetic lens, and this synthetic lens is connected to the object side. What is necessary is just to comprise from the object side lens which has a negative power to which the concave surface was turned to, and the image side lens which has a positive power.

  The imaging lens of the present invention is a three-group three-lens configuration or a three-group four-lens configuration. By making the first lens negative, a stable wide-angle lens can be obtained and at the same time a predetermined back focus is secured. can do. Further, the aberration correction can be satisfactorily performed by the correction lens of the third lens. Therefore, according to the present invention, a compact, lightweight and compact photographing lens corresponding to mega pixels on the order of mega can be manufactured at low cost.

  Embodiments of an imaging lens to which the present invention is applied will be described below with reference to the drawings.

  FIG. 1 is a configuration diagram illustrating the imaging lens of the first embodiment. The imaging lens 10 includes a first lens 1, a second lens 2, and a third lens 3 arranged in order from the object side. The first lens 1 is a meniscus lens having a negative power with a convex surface facing the object side. The second lens 2 is a lens having positive power with the concave surface facing the object side. The third lens 3 is a lens having negative power. The lens surfaces 1a, 1b, 2a, 2b, 3a, and 3b of the first lens 1, the second lens 2, and the third lens 3 are aspherical surfaces.

  In this example, an aperture stop 4 is disposed between the first lens 1 and the second lens 2, and a cover glass 5 is disposed between the third lens 3 and the imaging surface 6.

The lens data of the entire optical system of the imaging lens of Example 1 is as follows.
F number: 3.0
Focal length: F = 3.50mm
Focal length f1 of the first lens = −7.374 mm
Focal length f3 of the third lens = −3.172 mm
Total lens length: Σd = 5.693 mm (distance from the lens surface 1a of the first lens 1 to the imaging surface)

  Table 1A and Table 1B show the lens data of the lens surface of the imaging lens of Example 1 and the aspheric coefficient for defining the aspheric shape of the aspheric lens surface.

  In Table 1, i represents the order of the lens surfaces counted from the object side, R represents the radius of curvature of the lens surfaces, d represents the distance between the lens surfaces, Nd represents the refractive index of each lens, and νd represents Represents the Abbe number of each lens. In addition, a lens surface with a star on i is an aspherical surface.

  The aspherical shape adopted for the lens surface is as follows, assuming that the axis in the optical axis direction is X, the height in the direction orthogonal to the optical axis is H, the conical coefficient is k, and the aspherical coefficients are A, B, C, and D. It can be expressed by a formula.

  In this example, Σd / F = 5.693 / 3.50 = 1.627, | f3 / F | = 3.172 / 3.50 = 0.906, and f1 / F = −7.374. /3.50=−2.107, d1 / F = 0.75 / 3.50 = 0.214, which satisfies the conditional expressions (1) to (4). Further, since the air gap da (= d2 + d3) between the first lens 1 and the second lens 2 is 0.3 mm, da / F = 0.08, which is smaller than 0.3, and the conditional expression (5) is satisfied. Satisfies. Furthermore, the maximum emission angle of the chief ray is 30 degrees or less.

  2 is an aberration diagram illustrating various aberrations of the imaging lens of Example 1. FIG. 2A is a spherical aberration diagram showing spherical aberration SA, FIG. 2B is an astigmatism diagram showing astigmatism AS, and FIG. 2C is an aberration diagram showing distortion DIST. . T of astigmatism AS represents tangential, and S represents a sagittal image plane. FIG. 2D shows lateral aberration, DY is lateral Y aberration with respect to Y pupil coordinates, and DX is lateral X aberration with respect to X pupil coordinates. The meaning of these symbols is the same in the examples described later.

  Since the optical system of the imaging lens 20 of the second embodiment is basically the same as that of the imaging lens 10 of the first embodiment, the optical system will be described with reference to FIG. 1 showing the optical system of the first embodiment. The imaging lens 20 is a first lens 1 having a negative power with a convex surface facing the object side in order from the object side toward the imaging surface, and a second lens having a positive power with a concave surface facing the object side. The lens 2 and the third lens 3 having negative power are arranged. The lens surfaces of the first lens 1, the second lens 2, and the third lens 3 are aspherical surfaces. A cover glass 5 is disposed between the third lens 3 and the imaging plane 6.

The lens data of the entire optical system of the imaging lens 20 is as follows.
F number: 3.0
Focal length: F = 3.50mm
Focal length f1 of the first lens = -10.625 mm
Focal length f3 of the third lens = −3.157 mm
Total lens length: Σd = 5.655mm

  In this example, Σd / F = 5.655 / 3.50 = 1.616, | f3 / F | = 3.157 / 3.50 = 0.902, and f1 / F = -10.625. /3.50=−3.036 and d1 / F = 0.75 / 3.5 = 0.214, which satisfies the conditional expressions (1) to (4). Further, since the air gap da (= d2 + d3) between the first lens 1 and the second lens 2 is 0.3 mm, da / F = 0.08, which is smaller than 0.3, and the conditional expression (5) is satisfied. Is pleased. Furthermore, the maximum emission angle of the chief ray is 30 degrees or less.

  Tables 2A and 2B show lens data of each lens surface of the imaging lens 20 and aspherical coefficients for defining the aspherical shape of the aspherical lens surface. 3A to 3D show various aberrations of the imaging lens 20.

FIG. 4 is a schematic configuration diagram illustrating an imaging lens according to Example 3 of the present invention. The imaging lens 30 of this example includes, in order from the object side, a first lens group I including a first meniscus lens 11 having a positive power with a convex surface facing the object side, and a negative power with a concave surface facing the object side. A second lens group II composed of a composite lens 12 composed of an object side lens 12a having a positive power and an image surface side lens 12b having a positive power, and a third lens group III composed of a third lens 13 having a negative power. It is configured. The composite lens 12 constituting the second lens group II of this example is
This is a cemented lens of an object side lens 12a and an image plane side lens 12b made of a glass lens. The lens surfaces on both sides of the first lens 11, the lens surfaces on the image surface side of the image surface side lens 12b of the second lens group II, and the lens surfaces on both sides of the third lens 13 are aspheric. A cover glass 15 is disposed between the third lens group III and the imaging plane 16 as in the previous example.

The lens data of the entire optical system of the imaging lens 30 is as follows.
F number: 2.8
Focal length: F = 3.5mm
Focal length f1 of the first lens group = -11.208 mm
Focal length f3 of the third lens group = −5.363 mm
Total lens length: Σd = 6.812mm

  Tables 3A and 3B show the lens data of each lens surface of the imaging lens 30 and the aspherical coefficients for defining the aspherical shape of the aspherical lens surface. 5A to 5D show various aberrations of the imaging lens 30. FIG.

  In this example, Σd / F = 6.812 / 3.5 = 1.946, | f3 / F | = 5.363 / 3.5 = 1.532, and f1 / F = −11.208. /3.5=−3.212 and d1 / F = 0.7 / 3.5 = 0.2, which satisfies the conditional expressions (1) to (4). Further, since the air gap da (= d2 + d3) between the first lens group and the second lens group is 0.45 mm, da / F = 6.812 / 3.5 = 0.128, and from 0.3 And the conditional expression (5) is satisfied. Furthermore, the maximum emission angle of the chief ray is 30 degrees or less.

  In this example, the second lens group is a cemented lens, but it goes without saying that it may be a separate synthetic lens.

It is a schematic block diagram which shows the optical system of the imaging lens of Example 1 and 2 to which this invention is applied. It is an aberration diagram of the imaging lens of FIG. It is an aberration diagram of the imaging lens of Example 2 to which the present invention is applied. It is a schematic block diagram which shows the optical system of the imaging lens of Example 3 to which this invention is applied. FIG. 5 is an aberration diagram of the imaging lens in FIG. 4.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1, 11 1st lens 2 2nd lens 3, 13 3rd lens 12 Composite lens 12a Object side lens 12b Image surface side lens 4 Aperture 5, 15 Cover glass 6, 16 Imaging surface 10 Imaging lens 20 Imaging lens 30 Imaging lens d1, d4, d6 Lens thickness da Air spacing between the first lens and the second lens

Claims (7)

Having a first lens group, a second lens group and a third lens group arranged in order from the object side;
The first lens group includes a first lens having a negative power with a convex surface facing the object side,
The second lens group includes a second lens having a positive power with a concave surface facing the object side,
The third lens group includes a third lens having negative power,
Of the lens surfaces of the first and second lenses, at least one lens surface is aspheric.
The third lens has both aspheric surfaces,
The combined focal length of the imaging lens is F, the distance on the optical axis from the object-side lens surface of the first lens to the imaging plane is Σd, the focal length of the first lens is f1, and the focal length of the third lens Is f3,
0.5 <Σd / F <3.0 (1)
0.5 <| f3 / F | <1.6 (2)
−10 <f1 / F <−0.5 (3)
An imaging lens. In claim 1,
Σd / F <1.5
An imaging lens. In claim 1 or 2,
When the thickness of the first lens is d1,
0.1 <d1 / F <0.4 (4)
An imaging lens. In claim 3,
0.15 <d1 / F <0.3
An imaging lens. In any one of claims 1 to 4,
When the air gap formed by the first lens and the second lens is da,
da / F <0.3 (5)
An imaging lens. In any one of claims 1 to 5,
An imaging lens having a maximum chief ray emission angle of 30 degrees or less. In any one of claims 1 to 6,
The second lens group is a composite lens, and the composite lens is an imaging lens including an object side lens having a negative power with a concave surface facing the object side and an image surface side lens having a positive power.

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