JP2008083735A - Wide angle lens - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain a wide angle lens of satisfactory performance in visible light and near infrared light even though the lens is small and inexpensive and tolerance is also loose. <P>SOLUTION: The wide angle lens successively comprises a first lens 11 that is a double-concave lens having negative power, and a second lens 12 that is a double-convex lens from an object side. One or more lens surfaces are aspheric, and the wide angle lens satisfies conditions; 0.6D2/f≤¾f2/f1¾/≤2.3D2/f and 0.6h2≤r2≤1.0h2 when the focal length of the entire system is defined as f, the focal length of the first lens is defined as f1, the focal length of the second lens is defined as f2, the radius of curvature of the surface of the first lens on an image side is defined as r2, the effective radius on the surface of the first lens on the image side is defined as h2, and a distance between the surfaces of the first lens and the second lens is defined as D2. The wide angle lens requires only two lenses and is small, and also provides a very satisfactory formed image even in the near infrared light. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a wide-angle lens used for a door surveillance camera, a security camera, a surveillance camera, a video phone camera, or the like installed outdoors or indoors, particularly an ultra-wide-angle lens.

An imaging lens used for a monitoring camera such as a door monitoring camera or a video phone camera is required to be a super-wide-angle type wide-angle lens having a wide imaging area in addition to a resolving power.
Conventionally, such a lens has a large number of lenses and is difficult to reduce in size and cannot be manufactured at low cost.

  In addition, for some purposes, there are cases where there is no illumination light in the surroundings, for example, shooting with near-infrared light mounted on a camera, but for those designed with only visible light, near-infrared light illumination can be used. Shooting is blurred.

Japanese Patent No. 2015317 discloses a two-group two-lens configuration. However, in this case, a meniscus lens is used as the first lens. For this reason, when downsizing, the image side of the first lens is arranged. It is necessary to make the curvature very tight, and there are problems such as weak tolerance for axis deviation and difficult to manufacture.
Japanese Patent No. 2015317

  The present invention has been invented in view of the above-described conventional problems, and provides a wide-angle lens that is small and inexpensive, and exhibits good performance in visible light and near-infrared light even if the tolerance is loose. This is a problem.

In order to solve the above problems, a wide-angle lens according to the present invention includes a first lens that is a biconcave lens having negative power in order from the object side, and a second lens that is a biconvex lens, and at least an image of the first lens. The lens surface on the side is an aspheric surface, the focal length of the entire system is f, the focal length of the first lens is f1, the focal length of the second lens is f2, the radius of curvature of the image side surface of the first lens is r2, When the effective radius on the image side surface of the first lens is h2, and the distance between the surfaces of the first lens and the second lens is D2, the following conditions are satisfied: 0.6D2 / f ≦ | f2 / f1 | ≦ 2.3D2 / f (iv)
0.6h2 ≦ r2 ≦ 1.0h2 (v)
It has the feature in satisfying.

  Since the first lens is a biconcave lens, the refractive power on the image side is distributed to the object side, so that the curvature on the image side can be weakened. Besides being strong, it is easy to process in terms of manufacturing. However, if the object side is a concave surface, the incident angle of a wide-angle light beam increases and the incident power tends to decrease, but this can be avoided by using a gentle curvature, a flat surface, or an aspherical surface.

  The condition (iv) here is for improving the performance when irradiated with near-infrared light. In general, in a lens with spherical aberration correction only in the visible light range, the spherical aberration for near-infrared light is greatly shifted to the plus side from the image point of visible light, but it is balanced with the image point of visible light. Therefore, the target performance is obtained by slightly overcorrecting the spherical aberration of visible light. Here, it is well known that spherical aberration is over in a concave lens and spherical aberration is under in a convex lens. However, in the present invention, the larger the distance D2 between the first lens and the second lens is, the larger the aberration occurs in the concave lens. The amount of spherical aberration over is emphasized.

  In the condition (iv), when the value of | f2 / f1 | is smaller than the value of 0.6D2 / f, the amount of overspherical aberration is too small, and the spherical aberration in the visible light region is good. However, the spherical aberration in the near-infrared light is too large, leading to a significant performance degradation. Conversely, when the value of | f2 / f1 | is larger than the value of 2.3D2 / f, the spherical aberration is excessively corrected, resulting in poor performance.

  Furthermore, the condition (v) is a condition for correcting spherical aberration satisfactorily in relation to the condition (iv), and when the spherical aberration is slightly overcorrected as described above, Considering the balance with the peripheral edge, the peripheral edge of the spherical aberration becomes abruptly overcorrected. In order to keep this point favorable, the surface on the image side (second surface) of the first lens is aspherical, and higher-order terms of spherical aberration are closed, and the spherical aberration at the peripheral portion is abruptly overcorrected. There is no such thing.

  In the condition (v), when the curvature radius r2 of the second surface is larger than the effective radius h2, the influence of the higher-order term is too small and the spherical aberration at the peripheral portion is corrected too much. It becomes difficult. On the other hand, when the radius is smaller than 0.6 times the effective radius h2, it is advantageous for correcting the spherical aberration at the peripheral portion, but the curvature radius r2 of the second surface becomes too small, making lens processing difficult.

Furthermore, the invention of claim 2 is in addition to the above-mentioned claim 1,
1.0 ≦ D2 / f ≦ 1.5 (vi)
It has the feature in satisfying. This condition is for improving the spherical aberration in relation to the condition (iv) and obtaining a required back focus. In an ultra-wide-angle lens, the focal length is set small, so the back focus of the lens also becomes small.If this back focus is not large enough, a filter or the like is placed between the lens and the image plane. In addition to being unable to insert, the exit angle of the imaging surface becomes large, leading to a reduction in the amount of peripheral light.

  In the condition (vi), if the value of D2 / f is smaller than the lower limit 1.0, the required back focus cannot be obtained, and if it is larger than the upper limit 1.5, it is advantageous for obtaining the back focus. This is not preferable because the lens system becomes too large.

  The present invention has a small number of lenses, ie, a set of two lenses, and the distance between the surfaces of the first lens and the second lens is small. Therefore, the present invention is inexpensive and small, and the refractive power of each lens is suppressed. Therefore, it is easy to process and strong against assembly tolerances. In addition, the near-infrared light that can obtain a very good image not only in the visible light but also in the near-infrared light with a fixed focus, and does not make the subject feel glare when the illumination is insufficient. Can be light.

  Further, in the invention of claim 2, since the number of lenses is as small as two sets and the surface distance between the first lens and the second lens is small, not only is it inexpensive and small, but also the required back focus can be secured. It is also possible to insert a filter for color correction.

A first lens that is a biconcave lens having negative power in order from the object side and a second lens that is a biconvex lens, one or more lens surfaces being aspherical, and a radius of curvature of the second lens on the object side Is r3, the radius of curvature of the image side of the second lens is r4, the distance between the surfaces of the first lens and the second lens is D2, and the focal length of the entire system is f, the following condition 1 <| r4 | / r3 <3 (i)
D2 <1.5f (ii)
If the above is satisfied, the following can be obtained.

  That is, since the first lens is a biconcave lens, the refractive power on the image side is distributed to the object side, and the curvature on the image side can be weakened. In addition to being resistant to misalignment, it is easy to process in terms of manufacturing. However, if the object side is a concave surface, the incident angle of a wide-angle light beam increases and the incident power tends to decrease, but this can be avoided by using a gentle curvature, a flat surface, or an aspherical surface.

  Conditional expression (i) is a conditional expression for suppressing the refractive power of the second lens when the inter-surface distance D2 is shortened. When the lower limit is exceeded, the curvature r4 on the image side of the second lens must be very strong in order to perform sufficient correction even if the inter-plane distance D2 is short, and the axis increases when the curvature is increased. It becomes weak to assembly tolerances such as deviation. In particular, when the stop is arranged behind the second lens, the effective lens diameter is reduced, and the second lens must be thickened to ensure brightness, and the total length is increased, which is not desirable. On the other hand, when the upper limit is exceeded, the curvature of the second lens on the object side becomes strong, and it becomes weak against assembly tolerances, particularly axis deviation.

  Conditional expression (ii) realizes downsizing while keeping each aberration good. The lower limit is preferably about 0.5f <D2. Exceeding the upper limit undesirably increases the overall length.

When the focal length of the second lens is f2,
0.7f <f2 <1.5f (iii)
It is more preferable to satisfy the above condition.

  The condition (iii) determines the power distribution of the entire system, and is a condition for keeping the inter-surface distance D2 between the first lens and the second lens within the range of the conditional expression (ii). In particular, when the focal length f of the entire system is small, the range of f <f2 is favorable in order to suppress the power increase of each lens. However, when the focal length f of the entire system is relatively long, the range in which the size can be reduced and aberrations can be corrected appropriately is f2 ≦ f. If the lower limit is exceeded, the refractive power of the second lens increases, making it difficult to correct aberrations. On the other hand, if the upper limit is exceeded, the inter-surface distance D2 must be increased, and the total length becomes longer.

  A specific example satisfying the above conditions for a camera using an image sensor such as a CCD is shown in FIG. In the figure, 11 is a first lens on the object side, 12 is a second lens on the image side, 13 is a stop disposed on the image side of the second lens 2, 14 is a cover glass of an image sensor such as a CCD, and 15 is a CCD. The wide-angle lens is composed of a first lens 11 that is a biconcave lens and a second lens 12 that is a biconvex lens having a positive power, as is apparent from the drawing. .

  The optical arrangement of the example of FIG. The radius of curvature of the i-th surface from the object side is Ri (i = 1 to 7), the surface interval is Di (i = 1 to 7) (where the fifth surface is the aperture position), and j = 1. Reference numeral 2 denotes a first lens and second lens, and j = 3 denotes a cover glass 14, and the refractive index of the d-line of each material is Nj (j = 1 to 3). The surface marked with # is an aspherical surface, and its conical coefficient K and aspherical coefficient A are shown in Table 2.

  The aspherical surface is represented by the following known aspherical expression, where X is the coordinate in the optical axis direction with the intersection with the optical axis as the origin, and Y is the coordinate in the direction passing through the origin and perpendicular to the optical axis. .

X = [CY ² / {1+ (1- (K + 1) (CY) ² ) ^1/2 }] + AY ⁴ + ...
However, C = 1 / Ri
This wide-angle lens has | r4 | /r3=1.75, D2 = 1.4f, f2 = 1.33f, and satisfies all the conditional expressions (i), (ii), and (iii).

  FIG. 2 shows the spherical aberration, astigmatism, and distortion of visible light in the optical characteristics of the wide-angle lens, and FIG. 3 shows the lateral aberration. In the figure, C is the C-line characteristic, d is the d-line characteristic, F is the F-line characteristic, m is the meridional plane, and s is the sagittal plane.

  FIG. 4 shows another example. The basic configuration is the same as that described above, and includes a first lens 11 that is a biconcave lens and a second lens 12 that is a biconvex lens having positive power. The optical arrangement of this is shown in Table 3, and the aspherical cone coefficient K and the aspherical coefficient A marked with # are shown in Table 4.

  This wide-angle lens also satisfies | conditions (i), (ii), and (iii) because | r4 | /r3=1.1, D2 = 0.63f, and f2 = 0.95f. . FIG. 5 shows the spherical aberration, astigmatism, and distortion of visible light in the optical characteristics, and FIG. 6 shows the lateral aberration. In the figure, C is the C-line characteristic, d is the d-line characteristic, F is the F-line characteristic, m is the meridional plane, and s is the sagittal plane.

  In the above two examples, each aberration when the near-infrared light illumination wavelength is 940 nm is well corrected at the same focal position, and the near-infrared light also has good characteristics. It was to show.

  FIG. 7 shows an example according to the present invention. The basic configuration is the same as that described above, and is a first lens 11 that is a biconcave lens and a second lens 12 that is a biconvex lens having a positive power, and has a maximum field angle of 130 °. The four surfaces of the first and second surfaces 11 and the first and second surfaces of the second lens 12 are aspherical.

  The optical arrangement is shown in Table 1. The radius of curvature of the i-th surface from the object side is Ri (i = 1-7), the surface interval is Di (i = 1-7), and the effective radius is hi (i = 1-5) (provided that The surface is the aperture position), j = 1 and 2 are the first and second lenses, respectively, j = 3 is the cover glass 14 of the CCD, and the refractive index of the d-line of each material is Nj (j = 1 ~ 3). The surface marked with # is an aspherical surface, and its conic coefficient K and the aspherical coefficient ARk (k = 3, 4, 6, 8, 10) of the k-th power are shown in Table 2.

  The aspherical surface is expressed by the following known aspherical expression, where X is the coordinate in the optical axis direction with the intersection with the optical axis as the origin, and Y is the coordinate in the direction passing through the origin and orthogonal to the optical axis. .

X = [CY ² / {1+ (1- (K + 1) (CY) ² ) ^1/2 }] + ΣARY ^k
However, C = 1 / Ri
This wide-angle lens (super-wide-angle lens) has | f2 / f1 | = 0.788 (d2 / f), r2 = 0.984hi, (d2 / f) = 1.255, and the conditional expression (iv), All of (v) and (vi) are satisfied.

  The spherical aberration, astigmatism, and distortion of visible light in the optical characteristics of the wide-angle lens are shown in FIG. 8, the lateral aberration is shown in FIG. 9, and the spherical aberration and astigmatism in near-infrared illumination light having a wavelength of 940 nm. Aberration and distortion are shown in FIG. 10, and lateral aberration is shown in FIG. In the figure, C is the C-line characteristic, d is the d-line characteristic, F is the F-line characteristic, m is the meridional plane, and s is the sagittal plane.

  FIG. 12 shows another example. The basic configuration is the same as that described above, and includes a first lens 11 that is a biconcave lens and a second lens 12 that is a biconvex lens having positive power, and the maximum field angle is 130 °. The optical arrangement of this is shown in Table 3, and the aspherical conical coefficient K and the k-th aspherical coefficient ARk (k = 3, 4, 6, 8) marked with # are shown in Table 4.

  This super wide-angle lens also has | f2 / f1 | = 0.722 (d2 / f), r2 = 0.789 hi, (d2 / f) = 1.659, and the conditional expressions (iv), (v), All of (vi) are satisfied. In addition, the spherical aberration, astigmatism, and distortion of visible light in the optical characteristics are shown in FIG. 13, the lateral aberration is shown in FIG. 14, and the spherical aberration, astigmatism, and distortion in near-infrared illumination light having a wavelength of 940 nm. The aberration is shown in FIG. 15, and the lateral aberration is shown in FIG. In the figure, C is the C-line characteristic, d is the d-line characteristic, F is the F-line characteristic, m is the meridional plane, and s is the sagittal plane.

  In the above two examples, it can be seen that each aberration including spherical aberration is well corrected not only in the visible light region but also in the near infrared region.

It is the optical arrangement | positioning figure which showed the optical path of an example of embodiment of this invention simultaneously. It is a spherical aberration, astigmatism, distortion aberration figure same as the above. It is a lateral aberration figure same as the above. It is an optical arrangement | positioning figure which showed the optical path in other examples same as the above simultaneously. It is a spherical aberration, astigmatism, distortion aberration figure same as the above. It is a lateral aberration figure same as the above. It is the optical arrangement | positioning figure which showed simultaneously the optical path of the other example of embodiment of this invention. It is a spherical aberration, astigmatism, distortion aberration figure in visible light same as the above. It is a transverse aberration figure in visible light same as the above. It is a spherical aberration, astigmatism, distortion figure with near infrared light same as the above. It is a transverse aberration figure in near infrared light same as the above. It is an optical arrangement | positioning figure which showed the optical path in other examples same as the above simultaneously. It is a spherical aberration, astigmatism, distortion aberration figure in visible light same as the above. It is a transverse aberration figure in visible light same as the above. It is a spherical aberration, astigmatism, distortion figure with near infrared light same as the above. It is a transverse aberration figure in near infrared light same as the above.

Explanation of symbols

11 First lens 12 Second lens

Claims (2)

1. A first lens that is a biconcave lens having negative power in order from the object side and a second lens that is a biconvex lens, and at least the image side lens surface of the first lens is an aspheric surface. 6D2 / f ≦ | f2 / f1 | /≦2.3D2/f
0.6h2 ≦ r2 ≦ 1.0h2
Where f is the focal length of the entire system
f1: Focal length of the first lens
f2: focal length of the second lens
r2: radius of curvature of the image side surface of the first lens
h2: effective radius on the image side surface of the first lens
D2: A wide-angle lens satisfying the distance between the first lens and the second lens. Next condition 1.0 ≦ D2 / f ≦ 1.5
The wide-angle lens according to claim 1, wherein:

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