JP2007279335A - Wide-angle zoom lens - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a wide-angle zoom lens achieving the reduction of the number of lenses, and the simplification of a lens group moving system when varying power and made excellent in mass productivity while contriving the shortening of the entire optical length and the high performance of a lens. <P>SOLUTION: The wide-angle zoom lens comprises a first lens having negative refractive power and a second lens group having positive refractive power in order from an object side, and varies power by changing a distance between the first lens group and the second lens group. The first group includes only a cemented lens comprising a positive lens and a negative lens or a negative single lens, and the second group includes only a cemented lens comprising a positive lens and a negative lens, and at least the surface of the second group nearest to an image side is aspherical. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention mainly relates to an imaging lens for an imaging apparatus using a solid-state imaging device such as a CCD or a CMOS, such as a digital camera, a mobile phone camera, and a surveillance camera. The present invention relates to a wide-angle zoom lens having a total angle of view of 65 degrees or more and a zoom ratio of 2 times or more.

  In recent years, the demand for a camera having excellent portability using a solid-state image sensor such as a CCD or CMOS, such as a digital still camera or a camera for a mobile phone, has been remarkably increased. As such a lens for a camera, a demand for a zoom lens is increasing. However, since these lenses need to be mounted in a limited space, they are smaller than conventional lenses for a silver salt camera. There is a need. On the other hand, in order to meet the demand, it is necessary to have excellent mass productivity.

As prior art document information relating to this application, for example, Patent Document 1 is known.
Japanese Patent No. 3438552

  Conventionally, as a means for realizing a small zoom lens, a method of realizing a lens optical system with a small number of lenses has been adopted. As a design example, as shown in Japanese Patent No. 3438552, a zoom lens composed of two groups has been proposed in which the refractive index of each lens group is negative and positive from the object side. That is, when the lens group is the first group and the second group from the object side, the first group takes in light from a wide angle of view, and the second group has zooming and focusing functions.

  In general, in a zoom lens, when each group includes a plurality of lenses, the distance between lenses in the group and the eccentricity error between the lenses have a great influence on performance degradation. In order to reduce these effects, it is effective to use a cemented lens, and the above patent also shows a design example using a cemented lens in the second group. However, in the above-mentioned patent, a plurality of lenses are used in the first group, and the influence of the distance between these lenses and the eccentricity error on the performance deterioration is not sufficiently considered.

  In addition, when a cemented lens is used, the number of optical surfaces that can be used for light refraction decreases, and light refraction at the lens surface and light refraction at the cemented surface when the lens surface is in contact with air. In the latter case, the refractive power becomes weak. The reason is that the difference in the refractive index of the medium before and after the interface is small. Therefore, design restrictions are increased, which is disadvantageous in securing optical performance.

  In order to achieve the above object, the present invention has the following configuration.

  The first aspect of the present invention includes, in order from the object side, a first lens group having a negative refractive power and a second lens group having a positive refractive power, and the first lens group and the second lens group. The first lens group is composed of a cemented lens of a positive lens and a negative lens having a concave surface on the image side in order from the object side, and the second lens group Is composed of, in order from the object side, a cemented lens of a positive lens having a convex surface on the object side and a negative lens having a concave surface on the image side, and at least the most image side surface of the second lens group is an aspherical surface. To do.

  The zoom lens according to the present invention has a function of capturing light from a wide angle with a lens having negative refractive power constituting the first group, and performing zooming and focusing with the second group. At the time of zooming from the wide angle end to the telephoto end, it moves on the optical axis so that the first group and the second group approach each other. Since there are only two moving groups, the zoom function can be realized with a simple mechanism system.

  In addition, by adopting a configuration including only cemented lenses in both the first group and the second group, it is possible to realize a lens with small performance deterioration against manufacturing errors. In other words, in the case of a single lens in which each lens is not cemented, optical performance evaluation is impossible only after all the lenses are assembled, and the eccentricity of each lens in each group and the axial top surface spacing error. All contribute to performance degradation. When using a cemented lens, it is possible to compensate the accuracy of the cemented lens by applying an adhesive resin to the cemented surface and curing the resin while aligning the lenses. In the present invention, the first group is a single lens or one cemented lens and the second group is one cemented lens in the two-group zoom configuration. It is smaller than the eccentricity error between. Therefore, with the configuration of the present invention, it is possible to realize a zoom lens with reduced optical performance degradation due to manufacturing errors.

  Furthermore, in the zoom lens of the present invention, performance can be ensured even with a design in which only one surface of the second image surface of the second group is an aspherical surface. In general, as the number of aspheric surfaces increases, the degree of optical performance degradation increases with respect to manufacturing errors. However, in the present invention, the above-mentioned degradation in optical performance is suppressed by minimizing the use of aspheric surfaces.

  The invention according to claim 2 of the present invention satisfies the following conditions.

(1) ν ₁₂ −ν ₁₁ ≧ 15
(2) ν ₂₁ −ν ₂₂ ≧ 15
However,
ν ₁₁ : Abbe number of the positive lens in the first group ν ₁₂ : Abbe number of the negative lens in the first group ν ₂₁ : Abbe number of the positive lens in the second group ν ₂₂ : Abbe number of the negative lens in the second group Thus, by setting the Abbe number of the lenses constituting each group, it is possible to satisfactorily correct the longitudinal chromatic aberration in the first group and the lateral chromatic aberration in the second group.

  The invention according to claim 3 of the present invention is characterized in that the first lens group is composed of only a negative single lens having a concave surface with a strong curvature on the image side, and only one surface of the single lens is an aspherical surface. To do.

  As described above, by setting the first lens unit as a single lens and setting an aspherical surface, coma aberration due to off-axis light can be corrected and optical performance can be secured while reducing the number of lenses. In this case, if both surfaces are aspherical surfaces, the tolerance of decentering between the lens surfaces at the time of manufacturing the single lens becomes strict, and it becomes difficult to ensure the optical performance at the time of manufacturing.

  According to a fourth aspect of the present invention, in the zoom lens according to the third aspect, the following condition is satisfied.

(3) ν ₁₂ ≧ 50
(4) ν ₂₁ −ν ₂₂ ≧ 15
However,
ν ₁₂ : Abbe number of the negative lens in the first group ν ₂₁ : Abbe number of the positive lens in the second group ν ₂₂ : Abbe number of the negative lens in the second group The Abbe number as a lens material of the first group consisting of a single lens By using a material having a large value as expressed by equation (3), axial chromatic aberration can be corrected even with a single lens, and the second group as expressed by equation (4). By selecting the lens material, it is possible to satisfactorily correct the lateral chromatic aberration in the second group.

  The invention according to claim 4 of the present invention is characterized by satisfying the following conditions.

(5) 1.4 ≦ | f ₁ / f _w | ≦ 2.0
(6) 1.0 ≦ f ₂ / f _w ≦ 1.4
However,
f ₁ : Combined focal length of the first group f ₂ : Combined focal length of the second group f _w : Focal length at the wide angle end of the entire zoom lens Conditional expression (5) shows the range of the combined focal length of the first group It is determined. Although the combined refractive power of the first group is negative, many of them are caused by the concave surface of the negative lens image side surface. When the lower limit of conditional expression (5) is exceeded, it means that the combined refractive power of the first group becomes too strong, that is, the curvature of the concave surface of the negative lens image side surface constituting the first group becomes too strong. In this case, it is difficult to correct coma generated in the peripheral portion of the concave surface, and it is difficult to ensure the optical performance of the zoom lens. When the upper limit of conditional expression (5) is exceeded, it is disadvantageous for capturing light at the zoom wide-angle end, and it becomes difficult to ensure performance at the wide-angle end.

  Conditional expression (6) defines the range of the composite focal length of the second group. If the lower limit of conditional expression (6) is exceeded, it means that the combined refractive power of the second group becomes too strong. This means that the refractive power of the positive lens constituting the second group becomes too strong, but in this case, it becomes difficult to correct coma generated at the periphery of the positive lens, and the optical power of the zoom lens It is difficult to secure performance. If the upper limit of conditional expression (6) is exceeded, it means that the combined refractive power of the second group becomes too weak. In this case, the amount of movement of the second group accompanying zooming becomes large, and it is difficult to reduce the size of the entire lens system.

  According to a sixth aspect of the present invention, in the zoom lens according to the third aspect, a diffractive surface is provided on the outermost image side surface of the first lens group and the outermost object side surface of the second lens group.

  With the above configuration, chromatic aberration correction in each group can be performed more satisfactorily, and if the optical length is the same as compared with the case where no diffraction surface is used, the improvement in optical performance is maintained at the same level. Then, the optical total length can be shortened.

  The formula for the aspheric surface used in each example is shown below.

z = (h ² / r) / [1 + √ {1− (1 + K) (h / r) ² }] + A ₄ · h ⁴ + A ₆ · h ⁶ + A ₈ · h ⁸ + A ₁₀ · h ¹⁰ + A ₁₂ · h ¹²
However, here, the z-axis is set in the optical axis direction, and the x-axis and the y-axis are set in the directions orthogonal to the direction orthogonal to the optical axis. Furthermore, each parameter represents the following quantities.

h = √ (x ² + y ² )
r: paraxial radius of curvature, K: conical constant A _{p (p} = 4,6,8,10,12): aspheric coefficients of higher order Regarding the expression of K and A _p in the table, "E and its The “following number” represents “power of 10”, and the numerical value is multiplied by the immediately preceding numerical value. For example, “6.0023456E-4” represents 6.023456 × 10 ⁻⁴ .

(Embodiment 1)
As for the first embodiment of the zoom lens according to the present invention, (Table 1) to (Table 4) show numerical examples, FIG. 1 shows the lens configuration diagram, and FIG. 2 shows the various aberration diagrams. The arrows in FIG. 1 represent the movement of each lens group during zooming from the wide-angle end to the telephoto end. 2A, 2B, and 2C respectively show various aberrations at the wide-angle end, the intermediate focal length, and the telephoto end. 3A, 3B, and 3C show astigmatism at the wide-angle end, the intermediate focal length, and the telephoto end, respectively, when the entire first lens unit is moved 20 μm eccentrically in the −Y direction in FIG. 4 (a), 4 (b), and 4 (c) show the amount of change at the wide-angle end, the intermediate focal length, and the telephoto end, respectively, when the entire second lens unit is moved 20 μm eccentrically in the −Y direction in FIG. Represents the amount of astigmatism change. Each parameter in the tables and drawings represents the following quantities.

r: radius of curvature d: lens thickness or lens interval n _d : refractive index of d line ν _d : Abbe number of d line.

    The unit of r and d is mm.

  This embodiment is a design example of a zoom lens compatible with a 1/4 inch CCD and having a zoom ratio of 2 ×, but the first group is a cemented lens of positive and negative lenses, and each surface is a spherical surface. . The second group is a cemented lens of positive and negative lenses, and only the concave surface closest to the image side is an aspherical surface. The diaphragm is installed behind the second group. Further, the optical characteristics of the lens materials are selected so that the Abbe number difference of each lens in the first group is 16.9, and the Abbe number difference of each lens in the second group is 15.8. The correction of chromatic aberration is made. Thus, although each group is composed of only cemented lenses and the aspherical surface is only one surface, each aberration is sufficiently corrected as a zoom lens. 3 and 4, it can be seen that the amount of astigmatism change is small over the entire image plane even when each group is individually moved eccentrically. In general, when the amount of change in astigmatism is large as the lens is decentered, the difference in resolution between the sagittal direction and the meridional direction of the image plane is enlarged, resulting in noticeable image quality degradation as an image flow phenomenon. In the zoom lens of this design, the amount of change in astigmatism is small even when decentering of each lens group occurs at the time of lens assembly or zooming, so that image quality deterioration can be suppressed.

(Embodiment 2)
With respect to Embodiment 2 of the zoom lens according to the present invention, (Table 5) to (Table 8) show numerical examples, and FIG. 5 shows various aberration diagrams. Since the lens configuration diagram is substantially the same as the lens configuration diagram in the first embodiment, a description thereof will be omitted. Further, as in the first embodiment, the amount of astigmatism change when the first group and the second group are each decentered by 20 μm in the −Y direction in FIG. c) and FIGS. 7 (a), (b) and (c), respectively.

  This example is a result of performing an optimum design based on the first embodiment while increasing the zoom ratio to 2.5 times. Also in this case, each aberration is sufficiently corrected as a zoom lens, and the amount of astigmatism change, that is, the performance variation is sufficiently suppressed even when each group is decentered.

(Embodiment 3)
With respect to Embodiment 3 of the zoom lens of the present invention, (Table 9) to (Table 12) show numerical examples, FIG. 8 shows the lens configuration diagram, and FIG. 9 shows various aberration diagrams. 10A, 10B, and 10B, the astigmatism change amount when the first group and the second group are each decentered by 20 μm in the −Y direction of FIG. c) and FIGS. 11 (a), (b) and (c), respectively.

  The present example is a design example of a zoom lens in which the first group is changed to one concave meniscus lens having an aspheric image side surface based on the first embodiment, corresponding to a 1/4 inch CCD, and a zoom ratio of 2 ×. is there. In this case, the generation of chromatic aberration is suppressed even with a single lens by selecting a sufficiently large Abbe number of 59.1 as the lens material of the first group. In addition, by setting an aspherical surface in the first lens group, the amount of coma that tends to increase at the periphery of the lens in the case of a spherical surface is suppressed. As a result, each aberration is sufficiently corrected as a zoom lens, and the amount of astigmatism change, that is, the performance variation is sufficiently suppressed even when each group moves eccentrically.

(Embodiment 4)
Regarding Embodiment 4 of the zoom lens according to the present invention, (Table 13) to (Table 16) show numerical examples thereof, and FIG. 12 shows various aberration diagrams thereof. Since the lens configuration diagram is substantially the same as the lens configuration diagram in the third embodiment, a description thereof will be omitted. Further, as in the first embodiment, the amount of astigmatism change when the first group and the second group are each decentered by 20 μm in the −Y direction in FIG. c) and FIGS. 14 (a), 14 (b), and 14 (c), respectively.

  This example is the result of optimum design based on Embodiment 3 with the zoom ratio increased by 2.5 times. Also in this case, each aberration and performance fluctuation when each group moves eccentrically are sufficiently suppressed.

(Embodiment 5)
Regarding Embodiment 5 of the zoom lens according to the present invention, (Numerical Tables 17) to (Table 21) show numerical examples thereof, and FIG. The lens configuration diagram is omitted because it is substantially the same as the lens configuration diagram in the third embodiment. 16A, 16B, and 16B show the amount of astigmatism change when the first group and the second group are each decentered by 20 μm in the −Y direction in FIG. c) and FIGS. 17 (a), (b) and (c), respectively.

  In this example, based on the third embodiment, the first group image side surface is set as an aspherical diffractive surface, and the second group most object side surface is set as an aspherical diffractive surface. This is a design example.

  The characteristics of an optical element having a diffractive surface (hereinafter abbreviated as diffractive optical element) are that a normal lens material has a positive Abbe number, but has a negative Abbe number, and the partial dispersion ratio is also greatly different. The point is mentioned. By utilizing these characteristics, chromatic aberration correction can be effectively performed.

  The diffraction surface is represented by the following phase equation.

φ (h) = (2π / λ _o ) · ΣC _n h ²ⁿ
However, each parameter represents the following quantities.

φ: phase h: distance from the optical axis to the lens surface in a direction perpendicular to the optical axis λ _o : reference wavelength C _n : phase coefficient Generally, the optical performance deteriorates as the optical total length is shortened. By effectively performing aberration correction as described above, it is possible to ensure sufficient performance even with a short optical total length, and in Embodiment 3, the optical total length is 15 mm, whereas an optical total length of 12 mm is realized. . Also in this case, each aberration and performance fluctuation when each group moves eccentrically are sufficiently suppressed.

  Regarding lens design parameters, Table 22 shows values in all the embodiments. All satisfy the scope shown in the claims, and the effect appears as good optical performance as shown in the embodiment. In all lens designs, the diagonal length of the image sensor is 4.7 mm, which corresponds to a 1/4 inch CCD. The maximum optical total length of the lens is sufficiently small when using only a refractive optical element, 15 mm when the zoom ratio is double, 17 mm when the zoom ratio is double, and 12 mm when using the diffractive optical element. The change range of the total angle of view by zooming is about 67 degrees to about 34 degrees in the first, third, and fifth embodiments, and about 67 degrees to about 27 degrees in the second and fourth embodiments. It has a sufficient zooming effect even in the wide-angle and telephoto range while including the angle of view of 50 to 60 degrees used.

  The imaging lens of the present invention is a zoom lens suitable for a solid-state imaging device, while ensuring optical performance, optical performance deterioration due to manufacturing errors is reduced, a zooming mechanism is simple, and the entire image at the wide angle end is reduced. It is useful for a small-sized wide-angle zoom lens with an angle of 65 degrees or more, a zoom ratio of 2 times or more, and compatible with an image sensor of 1/4 inch or more.

1 is a lens configuration diagram of an imaging lens according to Embodiment 1 of the present invention. Various aberration diagrams of the imaging lens in the same embodiment The figure showing the astigmatism movement amount on the image plane when the first group is moved by 20 μm eccentrically in the same embodiment The figure showing the astigmatism movement amount on the image plane when the second group is moved by 20 μm eccentrically in the same embodiment Various aberration diagrams of the imaging lens according to Embodiment 2 of the present invention The figure showing the astigmatism movement amount on the image plane when the first group is moved by 20 μm eccentrically in the same embodiment The figure showing the astigmatism movement amount on the image plane when the second group is moved by 20 μm eccentrically in the same embodiment The lens block diagram of the imaging lens in Embodiment 3 of this invention Various aberration diagrams of the imaging lens in the same embodiment The figure showing the astigmatism movement amount on the image plane when the first group is moved by 20 μm eccentrically in the same embodiment The figure showing the astigmatism movement amount on the image plane when the second group is moved by 20 μm eccentrically in the same embodiment Various aberration diagrams of the imaging lens according to Embodiment 4 of the present invention The figure showing the astigmatism movement amount on the image plane when the first group is moved by 20 μm eccentrically in the same embodiment The figure showing the astigmatism movement amount on the image plane when the second group is moved by 20 μm eccentrically in the same embodiment Various aberration diagrams of the imaging lens according to the fifth embodiment of the present invention The figure showing the astigmatism movement amount on the image plane when the first group is moved by 20 μm eccentrically in the same embodiment The figure showing the astigmatism movement amount on the image plane when the second group is moved by 20 μm eccentrically in the same embodiment

Explanation of symbols

S Diaphragm L1 1st lens L2 2nd lens L3 3rd lens L4 4th lens C Cover glass G1 1st lens group G2 2nd lens group I Image surface

Claims (6)

In order from the object side, a first lens group having a negative refractive power and a second lens group having a positive refractive power, and by changing the air gap between the first lens group and the second lens group The first lens group includes a cemented lens of a positive lens in order from the object side and a negative lens having a concave surface on the image side, and the second lens group has a convex surface on the object side in order from the object side. A zoom lens comprising a cemented lens of a positive lens having a negative lens and a negative lens having a concave surface on the image side, wherein at least the most image side surface of the second lens group is an aspherical surface. The zoom lens according to claim 1, wherein the following condition is satisfied.
(1) ν ₁₂ −ν ₁₁ ≧ 15
(2) ν ₂₁ −ν ₂₂ ≧ 15
However,
ν ₁₁ : Abbe number of the positive lens in the first group ν ₁₂ : Abbe number of the negative lens in the first group ν ₂₁ : Abbe number of the positive lens in the second group ν ₂₂ : Abbe number of the negative lens in the second group 2. The zoom lens according to claim 1, wherein the first lens unit includes only a negative single lens having a concave surface with a strong curvature on the image side, and only one surface of the single lens is an aspherical surface. The zoom lens according to claim 3, wherein the following condition is satisfied.
(3) ν ₁₂ ≧ 50
(4) ν ₂₁ −ν ₂₂ ≧ 15
However,
ν ₁₂ : Abbe number of the negative lens in the first group ν ₂₁ : Abbe number of the positive lens in the second group ν ₂₂ : Abbe number of the negative lens in the second group 5. The zoom lens according to claim 1, wherein the zoom lens satisfies the following conditions.
(5) 1.4 ≦ | f ₁ / f _w | ≦ 2.0
(6) 1.0 ≦ f ₂ / f _w ≦ 1.4
However,
f ₁ : Composite focal length of the first group f ₂ : Composite focal length of the second group f _w : Focal length at the wide angle end of the entire zoom lens system 4. The zoom lens according to claim 3, further comprising diffractive surfaces on the outermost image side surface of the first lens group and the outermost object side surface of the second lens group.

Images