JP2012234099A - Imaging lens - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a wide angle imaging lens satisfactorily correcting aberrations, being compact, light-weight and inexpensive, and having excellent optical performance.SOLUTION: The imaging lens includes three lenses, namely, in order from the object side, a first lens 110 having a negative refractive power, a second lens 120 having a positive refractive power, an aperture stop 130, and a third lens 140 having a positive refractive power. The first lens 110 has a meniscus shape with its convex surface facing the object side and an aspherical shape on the image surface side. The second lens 120 has a meniscus shape with its convex surface facing the image side and an aspherical shape on at least one of the surfaces. The third lens 140 has a biconvex shape and an aspherical shape on at least one of the surfaces. The focal lengths and the Abbe numbers of the lenses are appropriately set.

Description

  The present invention relates to an imaging lens used in an imaging apparatus equipped with a solid-state imaging device, such as a monitoring camera or a vehicle-mounted camera, and more particularly to a single-focus wide-angle imaging lens.

  Imaging lenses used for surveillance cameras and in-vehicle cameras are required to have good imaging performance over the entire screen while ensuring a wide angle of view. In addition, since the mounting space is often limited, it is required to be small and lightweight.

  The following Patent Documents 1, 2, and 3 have been proposed as single-focus wide-angle imaging lenses that may be able to meet these demands. However, the single focus lens described in Patent Document 1 is a wide-angle imaging lens in which the number of constituent lenses is reduced, and the size and weight are reduced. However, aberration correction is not sufficient, and a high optical performance surface is provided over the entire screen. I couldn't be satisfied. In addition, in the single focus lenses described in Patent Documents 2 and 3 that overcome this problem, the number of lenses is four and high imaging performance can be provided. However, further downsizing is desired now. .

JP 2003-195161 A JP 2008-268268 A JP 2009-8867 A

  The present invention has been made in view of the above points, and an object of the present invention is to appropriately set the shape of the lens, the shape of the aspherical surface, etc. while being small, light and inexpensive by the three-piece structure. Accordingly, a wide-angle imaging lens having high optical performance is provided.

  In order to achieve the above object, a lens according to a first invention has, in order from the object side, a first lens having a negative refractive power, a second lens having a positive refractive power, an aperture stop, and a positive refractive power. The first lens has a meniscus shape with a convex surface facing the object side and an aspherical shape on the image side, and the second lens has a convex surface on the image side. The directed meniscus shape and at least one surface has an aspheric shape, the third lens has a biconvex shape and at least one surface has an aspheric surface, and the following conditional expressions (1) to (5) ) Is satisfied.

1.0 <f2 / f3 (1)
1.0 <| f1 / f23 | <3.0 (2)
ν1> 50 (3)
ν2 <30 (4)
ν3> 50 (5)
However,
f1: focal length of the first lens f2: focal length of the second lens f3: focal length of the third lens f23: combined focal length of the second lens and the third lens ν1: Abbe number ν2 of the first lens with respect to the d-line: Abbe number ν3 for the d-line of the second lens: Abbe number for the d-line of the third lens.

  Conventionally, in a wide-angle lens having four lenses as in Patent Document 2 and Patent Document 3, two lenses having a strong negative refractive power, a lens having a large chromatic dispersion and a positive refractive power, and the rear of the aperture stop are used. By arranging a lens having a positive refractive power, the field curvature and the chromatic aberration of magnification were corrected satisfactorily.

  In the present invention, the image side surface of the first lens has a strong refractive power, and by appropriately setting the lens shape corresponding thereto, it is possible to reduce the size and weight while correcting various aberrations such as chromatic aberration and curvature of field. Can be achieved.

  Furthermore, all the first to third lenses constituting the imaging lens are formed of a resin material.

  According to the present invention, it is possible to provide a wide-angle imaging lens in which various aberrations are satisfactorily corrected with a three-lens configuration while being small, light, and inexpensive. As a result, a compact wide-angle imaging lens that can be mounted on a surveillance camera or a vehicle-mounted camera can be realized.

It is a figure which shows the basic composition of the imaging lens of this embodiment. In this embodiment, it is a figure which shows the aperture | diaphragm | squeeze part of an imaging lens, and the surface number provided with respect to each lens. FIG. 4 is an aberration diagram showing spherical aberration and astigmatism in Example 1. 6 is a diagram illustrating a configuration of an imaging lens employed in Example 2. FIG. In Example 2, it is an aberrational figure which shows spherical aberration and astigmatism. 6 is a diagram illustrating a configuration of an imaging lens employed in Example 3. FIG. In Example 3, it is an aberrational figure which shows spherical aberration and astigmatism. In Example 2 and Example 3, it is a figure which shows the paraxial image surface position for every temperature.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. FIG. 1 shows the lens configuration of the embodiment in an optical section. In these embodiments, in order from the object side, the first lens 110, the second lens 120, the aperture stop 130, the third lens 140, the cover glass 150, a CCD (Charge Coupled Device), a CMOS (Complementary Mental-Oxide Semiconductor device), etc. This is a single-focus lens 100 having a three-lens configuration in which the image sensor 160 is arranged.

  In the imaging lens embodying the present invention, the three lenses are, in order from the object side, a first lens 110 having a negative refractive power, a second lens 120 having a positive refractive power, an aperture stop 130, and a positive lens. They are arranged like a third lens 140 having refractive power. With a wide-angle lens, it is necessary to shorten the focal length in order to obtain a wide angle of view, but the back focus must be longer than the focal length due to mechanical limitations. Therefore, a lens having a negative refractive power is arranged in the front, once incident light is diverged, and then condensed by a lens having a positive rear refractive power, so that the principal point of the lens system is moved backward. It is possible to ensure a long back focus compared to the focal length. Specifically, light is diverged by the negative first lens and condensed by the positive second lens and the third lens. By placing the negative lens closest to the object side, it is possible to obtain a negative refractive power sufficient to place the principal point behind, and by placing a lens with a positive refractive power in front of the aperture stop, The chromatic aberration is favorably corrected, and a lens having a positive refractive power is disposed after the aperture stop, thereby reducing the incident angle on the image plane and favorably correcting the aberration.

  In the imaging lens, the first lens 110 is a meniscus lens having a convex surface facing the object side, the second lens 120 is a meniscus lens having a convex surface facing the image side, and the third lens 140 is a biconvex lens. Since the first lens has a meniscus shape with a convex surface facing the object side, the incident angle of off-axis rays with respect to the first surface can be kept small, and the occurrence of aberration can be suppressed. Also, by making the image side surface of the first lens an aspherical surface, a strong refractive power can be obtained for off-axis rays, and the image surface can be corrected well. Since the second lens has a shape with a concave surface on the object side and a convex surface on the image side, it is possible to satisfactorily correct lateral chromatic aberration while arranging the principal point behind. When the third lens has convex surfaces on both sides and has an aspheric surface, the incident angle on the image plane can be reduced.

  In the imaging lens 100, the light incident from the object side OBJS is the object side R1 surface 1, the image surface side R2 surface 2 of the first lens 110, the object side R3 surface 3, the image surface side R4 surface 4 of the second lens 120, The surface 5 of the aperture stop 130, the object side R5 surface 6, the image surface side R6 surface 7 of the third lens 140, the object side R7 surface 8 and the image surface side R8 surface 9 of the cover glass 150 are sequentially passed to the image sensor 160. And condensed.

  In the imaging lens embodying the present invention, f1 is the focal length of the first lens 110, f2 is the focal length of the second lens 120, f3 is the focal length of the third lens 140, f23 is the second lens 120, and the third lens 140. If the combined focal length is, the conditional expressions (1) to (2) are satisfied.

1.0 <f2 / f3 (1)
1.0 <| f1 / f23 | <3.0 (2)
If the lower limit of 1.0 in the equation (1) is not reached, the distance of the exit pupil is shortened, and the incident angle of off-axis rays on the image plane is increased. When the upper limit of 3.0 in the expression (2) is exceeded, it is difficult to secure a back focus having a sufficient length compared to the focal length. If the value falls below the lower limit 1.0 of the expression (2), it is difficult to reduce the size of the lens system. Moreover, since the curvature radius of each lens becomes small, processing becomes difficult.

  In the imaging lens embodying the present invention, the Abbe number with respect to the d-line of the material constituting the first lens 110 is 50 or more, the Abbe number with respect to the d-line of the material constituting the second lens 120 is 30 or less. The Abbe number with respect to the d line of the material constituting the three lenses 140 is set to 50 or more, respectively. As the Abbe number of the material constituting the first lens 110 that is the negative lens is closer to the object side than the aperture stop 130, the chromatic aberration of magnification generated in the first lens 110 becomes smaller and closer to the object side than the aperture stop 130. Yes, the smaller the Abbe number of the material constituting the second lens 120, which is a positive lens, the greater the chromatic aberration of magnification that occurs in the second lens 120. Therefore, the chromatic aberration of magnification that occurs in the first lens 110 and the third lens 140 is better. Can be corrected. This is also because the chromatic aberration of magnification can be reduced as the Abbe number of the material constituting the third lens 140, which is a positive lens, is closer to the image side than the aperture stop 130.

  In the imaging lens embodying the present invention, all the first to third lenses are preferably made of a resin material. By being formed of a resin material, weight reduction and cost reduction can be realized, and an aspherical shape can be easily manufactured. In addition, the movement of the image plane position due to a temperature change can be suppressed smaller than when a glass material is used for only one of the lenses.

More preferably, in the imaging lens embodying the present invention, the shape of the object side surface of the second lens 120 satisfies the following conditional expression (6). By taking a shape that satisfies the range of the expression (6), it becomes easy to correct the field curvature and the chromatic aberration of magnification with a good balance while shortening the distance between the first lens 110 and the second lens 120.
0.3 <(S _A 2−S _S 2) / S _A 2 <0.8 (6)
S _A 2: Sag amount at the effective diameter position of the object side surface of the second lens 120 S _A 2: Sag amount at the effective diameter position of the spherical surface having the center curvature of the object side surface of the second lens 120.

Furthermore, in the imaging lens embodying the present invention, it is preferable that the shape of the image side surface of the third lens 140 satisfies the following conditional expression (7). By taking a shape that satisfies the range of the expression (7), it becomes possible to set the distortion aberration appropriately while bringing the principal point position close to the image plane.
−0.4 <(S _A 3−S _S 3) / S _A 3 <−0.1 (7)
S _A 3: Sag amount at the effective diameter position of the image side surface of the third lens 140 S _S 3: Sag amount at the effective diameter position of the spherical surface having the central curvature of the image side surface of the third lens 140.

  Examples 1 to 3 according to specific numerical values of the imaging lens are shown below. In the numerical examples 1 to 3, the focal length, F number, angle of view, image height, total lens length, and back focus (Bf) are as shown in Table 1 below. Similarly, in the numerical examples 1 to 3, the numerical data of the conditional expressions (1) to (5) are the values described in Table 2 below.

  The aspherical shape of the lens described in the following numerical examples is positive in the direction from the object side to the image plane side, k is a conical coefficient, A is a fourth-order aspheric coefficient, B Is a 6th-order aspheric coefficient, C is an 8th-order aspheric coefficient, and D is a 10th-order aspheric coefficient. h represents the height of the light beam, c represents the reciprocal of the central radius of curvature, and Z represents the depth from the tangent plane with respect to the surface vertex.

<Example 1>
The basic configuration of the lens system in Embodiment 1 is shown in FIG. 2, each numerical data (setting value) is shown in Tables 3 and 4, and aberration diagrams showing spherical aberration and astigmatism are shown in FIG. .

  As shown in FIG. 2, the first lens 110 has a meniscus shape with a convex surface facing the object side, the second lens 120 has a meniscus shape with a convex surface facing the image side, and a third lens disposed on the image side of the aperture stop 130. 140 has a biconvex shape. The first lens 110 has an aspheric surface on the image side surface, and the second lens 120 and the third lens 140 have an aspheric surface on both surfaces.

  Further, as shown in the figure, the distance between the R1 surface 1 and the R2 surface 2 which is the thickness of the first lens 110 is D1, and the distance between the R2 surface 2 of the first lens 110 and the R3 surface 3 of the second lens 120 is D2. The distance between the R3 surface 3 and the R4 surface 4 that is the thickness of the second lens 120 is D3, the distance between the R4 surface 4 of the second lens 120 and the surface 5 of the aperture stop 130 is D4, and the surface 5 of the aperture stop 130 is The distance between the R5 surface 6 of the third lens 140 is D5, the distance between the R5 surface 6 and the R6 surface 7 which is the thickness of the third lens 140 is D6, the R6 surface 7 of the third lens 140 and the R7 surface of the cover glass 150. 8 is D7, the distance between the R7 surface 8 and the R8 surface 9 that is the thickness of the cover glass 150 is D8, and the distance between the R8 surface 9 of the cover glass 150 and the imaging surface (imaging surface) 160 is D9. .

  Table 3 shows the stop corresponding to each surface number of the imaging lens in Example 1, the radius of curvature R, the interval D, the refractive index Nd, and the dispersion value νd of each lens. The symbol * in the table represents an aspheric surface (the same applies to the following examples). Table 4 shows the aspheric coefficient of the predetermined surface.

In Example 1, the sag amount S _A 2 is 0.041, the sag amount S _S 2 is 0.024, and (S _A 2−S _S 2) / S _A 2 is 0.41, (6 ) Is satisfied. In this embodiment, the sag amount S _A 3 is 0.747, the sag amount S _S 3 is 0.976, and (S _A 3 -S _S 3) / S _A 3 is −0.31, (7 ) Is satisfied.
Numerical example 1

  3A and 3B show spherical aberration and FIG. 3B shows astigmatism in Example 1, respectively. The vertical axis in FIG. 3B represents the image height on the imaging plane. In FIG. 3B, the solid line S represents the value of the sagittal image plane, and the broken line T represents the value of the tangential image plane. As can be seen from FIG. 3, according to the first embodiment, spherical and astigmatism aberrations are satisfactorily corrected, and an imaging lens having excellent imaging performance can be obtained.

<Example 2>
The basic configuration of the lens system according to Embodiment 2 is shown in FIG. 4, numerical data (setting values) are shown in Tables 5 and 6, and aberration diagrams showing spherical aberration and astigmatism are shown in FIG. .

  As shown in FIG. 4, the first lens 110 has a meniscus shape with a convex surface facing the object side, the second lens 120 has a meniscus shape with a convex surface facing the image side, and a third lens disposed on the image side of the aperture stop 130. 140 has a biconvex shape. The first lens 110 has an aspheric surface on the image side surface, and the second lens 120 and the third lens 140 have an aspheric surface on both surfaces.

  Table 5 shows the diaphragm corresponding to each surface number of the imaging lens in Example 2, the radius of curvature R, the interval D, the refractive index Nd, and the dispersion value νd of each lens. Table 6 shows the aspheric coefficient of the predetermined surface.

In Example 2, the sag amount S _A 2 is 0.247, the sag amount S _S 2 is 0.094, and (S _A 2−S _S 2) / S _A 2 is 0.62, and (6 ) Is satisfied. In this embodiment, the sag amount S _A 3 is 0.330, the sag amount S _S 3 is 0.405, and (S _A 3−S _S 3) / S _A 3 is −0.23, (7 ) Is satisfied.
Numerical example 2

5A and 5B, in Example 2, FIG. 5A shows spherical aberration, and FIG. 5B shows astigmatism, respectively. The vertical axis in FIG. 5B represents the image height on the imaging plane. As can be seen from FIG. 5, according to the second embodiment, spherical and astigmatism aberrations are satisfactorily corrected, and an imaging lens having excellent imaging performance can be obtained.
<Example 3>
The basic configuration of the lens system according to Embodiment 3 is shown in FIG. 6, the numerical data (setting values) are shown in Tables 7 and 8, and the aberration diagrams showing spherical aberration and astigmatism are shown in FIG. . Example 3 is an example in which the material of the first lens 110 is made of resin based on the same specifications as Example 2.

  As shown in FIG. 6, the first lens 110 has a meniscus shape with a convex surface facing the object side, the second lens 120 has a meniscus shape with a convex surface facing the image side, and a third lens disposed on the image side of the aperture stop 130. 140 has a biconvex shape. The first lens 110 has an aspheric surface on the image side surface, and the second lens 120 and the third lens 140 have an aspheric surface on both surfaces.

  Table 7 shows the stop corresponding to each surface number of the imaging lens in Example 3, the radius of curvature R, the interval D, the refractive index Nd, and the dispersion value νd of each lens. Table 8 shows the aspheric coefficient of the predetermined surface.

In Example 1, the sag amount S _A 2 is 0.281, the sag amount S _S 2 is 0.084, and (S _A 2−S _S 2) / S _A 2 is 0.70, (6 ) Is satisfied. In this example, the sag amount S _A 3 is 0.361, the sag amount S _S 3 is 0.421, and (S _A 3−S _S 3) / S _A 3 is −0.16, (7 ) Is satisfied.
Numerical Example 3

  7A and 7B, in Example 3, FIG. 7A shows spherical aberration, and FIG. 7B shows astigmatism. The vertical axis in FIG. 7B represents the image height on the imaging plane. As can be seen from FIG. 7, according to the third embodiment, various aberrations of spherical and astigmatism are corrected well, and an imaging lens having excellent imaging performance can be obtained.

  FIG. 8 shows the relationship between the temperature and the back focus in the second and third embodiments. The horizontal axis represents temperature, and the vertical axis represents the normalized air distance from the lens final surface to the paraxial image plane with a value at 20 degrees Celsius. As can be seen from the figure, the image plane movement due to temperature change is less in Example 3 using a resin material than in Example 2 using a glass material for the first lens 110, although the specifications are the same. An imaging lens with excellent performance can be obtained. Therefore, in the present invention, it is indicated that the first lens 110 is preferably formed of a resin material in the same manner as the second lens 120 and the fourth lens 140.

100, 100A to 100C ... Imaging lens 110 ... First lens
120 ... second lens 130 ... aperture stop 140 ... third lens 150 ... cover glass 160 ... imaging surface (imaging surface)

Claims (2)

In order from the object side, three lenses, a first lens having a negative refractive power, a second lens having a positive refractive power, an aperture stop, and a third lens having a positive refractive power, are arranged. The first lens has a meniscus shape with a convex surface facing the object side and an aspherical shape on the image surface side, and the second lens has a meniscus shape with a convex surface facing the image side and an aspheric surface on at least one surface An imaging lens having a shape, wherein the third lens has a biconvex shape and an aspherical surface on at least one of the surfaces, and satisfies the following conditional expressions (1) to (5).
1.0 <f2 / f3 (1)
1.0 <| f1 / f23 | <3.0 (2)
ν1> 50 (3)
ν2 <30 (4)
ν3> 50 (5)
However,
f1: focal length of the first lens f2: focal length of the second lens f3: focal length of the third lens f23: combined focal length of the second lens and the third lens ν1: Abbe number ν2 of the first lens with respect to the d-line: Abbe number ν3 for the d-line of the second lens: Abbe number for the d-line of the third lens   2. The imaging lens according to claim 1, wherein all of the first to third lenses constituting the imaging lens are formed of a resin material.

Images