JPH0520724B2 - - Google Patents

Description

[Detailed description of the invention]

<Industrial Application Field> The present invention relates to a wide-angle imaging lens used in facsimile devices, image scanner devices, and the like. <Conventional technology> Lenses for facsimile equipment and image scanner equipment require close to 100% aperture efficiency and good performance over the entire screen, so they are Gaussian type (consisting of 6 lenses).
Even with the Xenotar type (5-element configuration), the brightness was F5 and the angle of view was only 50°. <Problem to be solved by the invention> As a lens system that covers a wide angle of view, the retrofocus type with a negative lens in front is suitable for use in facsimile equipment and image scanner equipment because it can achieve an aperture efficiency of over 100%. It's advantageous. However, the retrofocus type has the following drawbacks. In other words, it is difficult to faithfully reproduce the original due to the occurrence of negative distortion, and it is also difficult to correct the divergence aberration caused by the preceding negative lens, which requires a high For example, it is difficult to obtain cost savings. The inventor first proposed Japanese Patent Application Laid-Open No. 11216/1986 as a wide-angle imaging lens that takes advantage of the retrofocus type's advantage of ``over 100% aperture efficiency'' and ``eliminates negative distortion and divergence.'' did. The invention according to the above proposal includes a negative meniscus lens convex toward the object side in the first group, and a positive, negative,
A positive three-element positive lens system, with a negative meniscus lens convex toward the image side in the third group, creates a nearly symmetrical structure with extremely good distortion and aperture efficiency of over 100%. Ensure a caliber ratio of 1:
4.5, and the angle of view is 60°. The object of the present invention is to further reduce the number of lenses since the lens proposed above has a five-element structure, so there is a limit to cost reduction, and to provide a lens with good optical characteristics. <Means and effects for solving the problem> In order to achieve the above object, the wide-angle imaging lens according to the present invention is constructed as shown in the lens configuration diagrams shown in FIGS. 1, 3, 5, and 7. In order from the object side, the first
It consists of three groups: a group, a second group, and a third group.
The second group consists of a negative meniscus single lens with a convex surface facing the object side, a front positive lens with a convex surface with a large curvature facing the object side, and a rear positive single lens with a convex surface with a large curvature facing the image side. The third group is a negative meniscus lens with a convex surface facing the image side, and the convex surface in contact with the air on the object side of the front positive lens of the second group and the concave surface in contact with the air on the object side of the third group are both aspherical. It is characterized by being configured. Note that in all of the lens configuration diagrams, a cover glass is shown on the image side of the imaging lens. The first lens group, which is a negative meniscus single lens convex toward the object side, has an aperture efficiency of 100% to improve the flatness of the image plane. The second group is a condensing lens system with a strong positive refractive index, and there is a lens between the front positive lens with a convex surface with a large curvature facing the object side and the rear positive single lens with a convex surface with a large curvature facing the image side. A spacing ring or a diaphragm can be provided, and the amount of peripheral light can be adjusted by selecting the size of this spacing and the position and shape of the spacing ring or diaphragm. By arranging the third group in a shape and position that are nearly symmetrical to the first group, the flatness of the image plane and distortion can be made extremely good. Since the second group of the lens according to the present invention does not include a negative lens and has strong positive refractive power, it is difficult to correct spherical aberration and coma with only the negative lenses of the first and third groups. Can not. The beam of light incident on the object-side convex surface of the front positive lens of the second group has a large width, and when making this surface aspheric, it is necessary to correct spherical aberration and coma aberration (particularly of the rays passing through the peripheral part of the front lens). The results are good and the effect is remarkable. The concave surface facing the object side of the negative meniscus lens of the third group is arranged symmetrically with the convex surface of the front positive lens of the second group facing the object side with respect to off-axis rays. When this concave surface is made aspherical, it is effective in improving coma aberration (particularly of light rays passing through the peripheral portion of the rear lens) and curvature of field. The aforementioned aspherical surface of the lens according to the invention is the ninth
It is also possible to construct the lens by bonding an aspherical layer made of a transparent material to the surface of a spherical glass as shown in the lens configuration diagrams shown in FIGS. In this case, the radius of curvature of the spherical glass lens and the radius of apex curvature of the aspherical layer bonded thereto may be the same or slightly different, and the axial thickness of the aspherical layer is also 1% or less of the focal length of the entire system. If so, it is possible. By satisfying the above configuration and conditions, the present invention can provide a wide-angle imaging lens with good spherical aberration, coma aberration, distortion aberration, and image plane flatness. In the wide-angle imaging lens according to the present invention, by further adding the following conditions, it is possible to obtain a lens with better performance. (a) 53<(ν ₂ +ν ₃ )/2 (b) ν ₂ ≦ν ₃ (c) ν ₁ >ν _4where ν ₁ is the Atsube number of the material of the negative meniscus single lens in the first group, and ν ₂ is the The Atsube number ν ₃ of the material of the front positive lens of the second group (excluding the transparent material in cases where an aspherical layer made of a transparent material is bonded to the object side convex spherical surface) is the rear positive single lens of the second group The Atsube number of the material ν ₄ is the Atsube number of the material of the third group negative meniscus lens (excluding the transparent material in cases where an aspherical layer made of a transparent material is bonded to the object side concave spherical surface) According to the present invention Although the second group of the wide-angle imaging lens has strong positive refractive power, it does not include a negative lens that is effective in correcting chromatic aberration. Condition (a) is for improving axial chromatic aberration, and if this condition is not met, axial chromatic aberration becomes large due to insufficient correction. Even when the convex surface facing the object side of the front positive lens of the second group is constructed by bonding an aspherical layer made of a transparent material to the surface of a spherical glass lens, the refractive power of the aspherical layer is Since it is small compared to the refractive power of the group, it has little effect on longitudinal chromatic aberration. Increasing the refractive index of the material of the front positive lens of the second group has a remarkable effect on correcting sagittal coma, but as the refractive index increases, dispersion also increases, making it difficult to correct chromatic aberration. Condition (b) determines the relationship between the Atsube numbers of the materials of the front positive lens of the second group and the rear positive single lens.Sagittal coma can also be improved by increasing the refractive index of the material of the front positive lens of the second group. Moreover, it is intended to improve both axial chromatic aberration and lateral chromatic aberration. Even when the convex surface facing the object side of the front positive lens of the second group is constructed by bonding an aspherical layer made of a transparent material to the surface of a spherical glass lens, the refractive power of the aspherical layer is very small. Therefore, it has little effect on chromatic aberration. When condition (b) is not met, the undercorrected chromatic aberration of the rear positive lens of the second group increases, and it becomes difficult to correct it even if the material of the third group is selected. Condition (c) is intended to improve chromatic aberration of magnification. When this condition is not met, there is a marked tendency for the imaging magnification for wavelengths shorter than the reference wavelength to become smaller. Even in the case where the concave surface facing the object side of the third group is constructed by bonding an aspherical layer made of a transparent material to the surface of a spherical glass lens, the refractive power of the aspherical layer is compared to the refractive power of the third group. Because it is so small, it has little effect on chromatic aberration of magnification. Note that it is more preferable to satisfy the following conditions. (d) 0.9f＜Σd＜1.2f (e) 0.4f＜Σd＜0.6f where, f: Composite focal length of the entire system Σd: Axial thickness of the entire system Σd: Axial thickness of the second group Condition (d ) is related to the size of the lens system. When the total length becomes shorter than this range, the lens system becomes smaller, the Gapetsval sum increases, and the effect of increasing the aperture efficiency decreases. If the total length becomes longer than this range, the lens system will become larger and the lens barrel will also become larger. Condition (e) concerns the size of the second group. If the second group is made shorter than this range, the lens system becomes smaller, but the flatness of the image plane deteriorates. Furthermore, if the second group becomes longer than this range, the lens system becomes large. <Example> Hereinafter, Examples 1 to 8 of the wide-angle imaging lens of the present invention will be described in detail. The meanings of symbols used in this explanation are as follows. f: Synthetic focal length of the entire system m: Imaging magnification r _i : Radius of curvature of a spherical surface or apex radius of curvature of an aspherical surface r _ip : An aspheric layer made of a transparent material is formed on the object side of the r _i surface of a spherical glass lens. Radius of aspheric curvature at the apex when bonding d _i : Sequentially the axial thickness of the lens, or air spacing d _ip : When bonding an aspheric layer made of transparent material to the object side of the axial thickness d _i of the spherical glass lens The axial thickness of the aspherical layer n _i : The refractive index of the lens material with respect to the d-line n _ip : The aspherical surface when an aspherical layer made of a transparent material is bonded to the object side of a spherical glass lens (refractive index n _i ) The refractive index for the d-line of the material of the layer ν _i : The Atsbe number ν _i of the lens material in sequence: The aspheric layer made of a transparent material when bonded to the object side of the spherical glass lens (Atsbe number ν _i ) Atsube number of the material d _c : Axial thickness of the image side cover glass n _c : Refractive index of the material of the image side cover glass for the d line Σd : Axial thickness of the entire system Σd : Axial thickness of the second group The angle of view is This is when the image height was unified to 12.1, and it is possible to enlarge it slightly. Next, the formula for the shape of the aspherical surface is: Conic constant A _2i : When taken as an aspheric coefficient, it is expressed as X=Ch ² /{1+√1−(1+) ^{2 2} }+ 〓 ⁱ⁼² A _2i h 2i. 1, 3, 5, 7, 9, 11, 13, and 15 are configurations of the first to eighth embodiments of the wide-angle imaging lens according to the present invention. FIG. Numerical values for each example are shown in Tables 1 to 8.

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[Table] Figure 2, Figure 4, Figure 6, Figure 8, Figure 10,
12, 14, and 16 are aberration curve diagrams of the first to eighth embodiments of the wide-angle imaging lens according to the present invention. In all of the embodiments of the wide-angle imaging lens according to the present invention, the imaging magnification m is -0.112. This means that the imaging magnification m=- is applicable to objects at a finite distance, even though the photographic lens embodiments are disclosed for objects at infinity.
It is not limited to 0.112. Of course, it is more preferable to optimize for changes in imaging magnification, but it is also possible to maintain a good aberration balance even if the distances between the first and second groups, and between the second and third groups are slightly changed. It is also an advantage of the present invention that this can be done. Of course, this case also falls within the range of conditions (d) and (e). <Effects of the Invention> As described above, the wide-angle imaging lens according to the present invention includes a first group of negative meniscus lenses with a convex surface facing the object side, a positive lens group with a strongly convex surface facing the object side, and a subsequent lens group. The second group consists of a positive lens with a strongly convex surface facing the image side, and the third group consists of a negative meniscus lens with a convex surface facing the image side. Basically, it is symmetrical with an extremely small number of lenses, 4 lenses. It consists of close to. This eliminates spherical aberration and comatic aberration by making the frontmost surface of the second group, where the width of the light beam is largest, aspheric, and by making the concave surface on the object side of the third group aspheric, It was possible to remove the residual coma aberration that could not be removed by the second group, and to improve the flatness of the image plane. In addition, the under-corrected chromatic aberration that occurs in the positive lens system of the second group is corrected by the negative meniscus lenses of the first and third groups, and chromatic aberration of magnification can also be removed depending on the size of the Atsube number. We were able to obtain good performance with an aperture efficiency of 100% at a wide angle of view of 60°.

[Brief explanation of the drawing]

1, 3, 5, 7, 9, 11, 13, and 15 are configurations of the first to eighth embodiments of the wide-angle imaging lens according to the present invention. FIG. Figure 2, Figure 4, Figure 6, Figure 8
Figures 10, 12, 14 and 16
The figures are aberration curve diagrams of the first to eighth embodiments of the wide-angle imaging lens according to the present invention. r _i ...The radius of curvature of the spherical surface or the apex radius of curvature of the aspheric surface, r _ip ...The apex radius of curvature of the aspheric surface when an aspheric layer made of transparent material is bonded to the object side of the r _i surface of the spherical glass lens , d _i ...sequentially the axial thickness of the lens or the air gap, d _ip ...the axis of the aspheric layer when an aspheric layer made of a transparent material is bonded to the object side of the spherical glass lens with the axial thickness d _i Top thickness, n _i ...
…Sequentially, the refractive index of the lens material for the d-line, n _ip
...Refractive index for the d-line of the material of the aspherical layer when an aspherical layer made of a transparent material is bonded to the object side of a spherical glass lens (refractive index n _i ), d _c ...axial thickness of the image-side cover glass , n _c ...Refractive index for the d-line of the material of the image side cover glass, Σd... Axial thickness of the entire system, Σd... Axial thickness of the second group.

Claims (1)

[Claims] 1. Three groups, in order from the object side: a first group, a second group, and a third group.
The first group is a negative meniscus single lens with a convex surface facing the object side, the second group is a front positive lens with a convex surface with a large curvature facing the object side, and a rear positive lens with a convex surface with a large curvature facing the image side. Consisting of a single lens, the third group is a negative meniscus lens with a convex surface facing the image side, and the convex surface of the front positive lens of the second group contacts the air on the object side, and the convex surface of the third group contacts the air on the object side. A wide-angle imaging lens characterized in that both concave surfaces are aspherical, and the Atsube number of the material of each lens satisfies the following conditions. (a) 53<(ν ₂ +ν ₃ )/2 (b) ν ₂ ≦ν ₃ (c) ν ₁ >ν _4where ν ₁ is the Atsube number of the material of the negative meniscus single lens in the first group, and ν ₂ is the The Atsube number of the material of the front positive lens of the second group ν ₃ is the Atsube number of the material of the rear positive single lens of the second group. ν ₄ is the Atsube number of the material of the negative meniscus lens of the third group. 2 From the object side: 1st group, 2nd group, 3rd group
The first group is a negative meniscus single lens with a convex surface facing the object side, the second group is a front positive lens with a convex surface with a large curvature facing the object side, and a rear positive lens with a convex surface with a large curvature facing the image side. Consisting of a single lens, the third group is a negative meniscus lens with a convex surface facing the image side, and the convex surface of the front positive lens of the second group that contacts the air on the object side and the air of the third group that contacts the object side. Both of the contacting concave surfaces are aspherical, and at least one of them is constructed by bonding an aspherical layer made of a transparent material to the object-side surface of a spherical glass lens, and the Abbe number of the material of each lens satisfies the following conditions. A wide-angle imaging lens characterized by: (a) 53<(ν ₂ +ν ₃ )/2 (b) ν ₂ ≦ν ₃ (c) ν ₁ >ν _4where ν ₁ is the Atsube number of the material of the negative meniscus single lens in the first group, and ν ₂ is the The Atsube number ν ₃ of the material of the front positive lens of the second group (excluding the transparent material in cases where an aspherical layer made of a transparent material is bonded to the object side convex spherical surface) is the rear positive single lens of the second group The Atsbe number of the material ν ₄ is the Atsbe number of the material of the third group negative meniscus lens (in the case of an aspherical layer made of a transparent material bonded to the object side concave spherical surface, the transparent material is not included).