JP2012203119A - Imaging optical system and imaging apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an imaging optical system and an imaging apparatus that are able to restrain occurrence of ghost in the imaging optical system having a lens composed of two lens parts, able to preferably further shorten the entire length of an optical system in addition to this, and able to achieve a size reduction as a whole.SOLUTION: The imaging optical system comprises two lenses, which are in order from an object side: a first positive power lens 11 of meniscus shape whose convex face is oriented toward the object side; and a positive power second lens. When the interval between the first lens and the second lens 12 is d2, a back focus when parallel light from the image surface side of the second lens is caused to enter is fb2, the wall thickness of the first lens is d1, the curvature radius of the first surface of the first lens is R1, the refraction index of the first lens is n1, and R1-d1*[1+(n1-1)/n1] is δ, the following conditional expression, d2<δ+fb2, is satisfied.

Description

  The present invention relates to an imaging optical system used for imaging light for capturing an image, and more particularly to an imaging optical system having a two-lens configuration lens. Furthermore, the present invention relates to an image pickup apparatus having an image pickup optical system used for, for example, an in-vehicle camera or a surveillance camera.

  In general, an imaging optical system used for imaging light is a combination of a plurality of lenses in order to obtain a sharp image with few aberrations. In cameras that require image performance, an imaging optical system having five or more lenses is often used. However, when the number of lenses is large, it is difficult to reduce the size and weight, and the production cost is not suitable for the cost reduction.

In recent years, in response to demands for downsizing and weight reduction of imaging devices such as in-vehicle cameras and surveillance cameras, downsizing of imaging elements such as CCDs and CMOSs has been achieved. There is a need for shortening and downsizing.
In view of this, an imaging optical system having a two-lens lens has been proposed and put to practical use as an imaging optical system in which the total length of the optical system is shortened.

  For example, Patent Document 1 (Japanese Patent Laid-Open No. 10-90597) discloses, in order from the object side, a first lens made up of a meniscus lens having a convex surface directed toward the object side, and a second made up of a biconvex lens having a positive refractive power. A configuration of a photographic lens having a lens is disclosed. The photographic lens disclosed herein further defines the relationship between the focal length of the entire lens system, the focal length of the first lens, and the spatial distance between the first lens and the second lens, so that the entire lens system is It is possible to correct distortion while minimizing.

  As a related technique, Patent Document 2 (Japanese Patent No. 3737095) discloses an aperture stop, a first lens composed of a meniscus lens having a convex surface facing the object side, and a convex surface facing the object side in order from the object side. A configuration of an imaging lens having a second lens composed of a directed meniscus lens is disclosed. The imaging lens disclosed herein is capable of correcting various aberrations by further defining the relationship such as the combined focal length of the imaging lens, the focal length of the first lens, and the focal length of the second lens, and optical. The length can be shortened.

JP-A-10-90597 Japanese Patent No. 3737095

  However, although the photographing lens disclosed in Patent Document 1 described above is effective for miniaturization, in a conventional imaging optical system having such a two-lens configuration, a light beam incident from outside the angle of view is a lens. There is a problem that ghosts are likely to occur due to repeated reflections. In order to suppress the occurrence of this ghost, it is conceivable to attach a mechanical hood or the like on the front surface (object side) of the imaging optical system so that rays outside the angle of view do not enter the lens. The total length becomes long and the outer diameter becomes large, which is contrary to the miniaturization that is the main purpose of the two-lens configuration lens.

On the other hand, in a conventional imaging optical system, a configuration in which an aperture stop is disposed in front of the first lens as disclosed in Patent Document 2 and the like is often used. For this reason, a space for installing an aperture stop is required in addition to the optical system, which is a problem of downsizing.
Therefore, in the present invention, it is possible to suppress the occurrence of a ghost in an imaging optical system having a two-lens configuration, and in addition to this, the overall length of the optical system can be further shortened, and the overall size can be reduced. It is an object to provide an imaging optical system and an imaging apparatus capable of achieving the above.

As a result of studying the principle of ghost generation in a two-lens configuration, the present inventors have found the following knowledge regarding the lens configuration for suppressing the generation of the ghost, and have made the present invention.
First, the principle of ghost generation is shown below.
As shown in FIG. 6, an imaging optical system having, as an example, an imaging optical system having a first lens 51 having a positive power with a convex surface facing the object side and a second lens 52 having a positive power in order from the object side. explain. In this imaging optical system, a cover glass 53 is disposed between the second lens 52 and the image plane 54.

Light L0 incident from outside the effective field angle of the imaging optical system enters the first lens 51 and is reflected by the R2 surface on the image plane side, and further reflected by the R1 surface on the object side. Then, the reflected light of the R1 surface is collected at the point O. When the point O is on the subject side from the back focus when the parallel light from the image plane 54 side of the second lens 52 is entered, the light traveling from the point O toward the image plane 54 converges on the image plane. It is considered that a ghost occurs.
Thus, the present inventors have found that the distance d between the first lens 51 and the second lens 52 greatly affects the occurrence of ghost, and proposes the following invention.

An imaging optical system according to the present invention has a two-lens configuration including, in order from the object side, a meniscus first lens having a positive power with a convex surface facing the object side, and a second lens having a positive power. It consists of a lens, the distance between the first lens and the second lens is d2, the back focus when the parallel light from the image plane side of the second lens is put in, fb2, the thickness of the first lens is d1, the thickness of the first lens When the radius of curvature of the first surface is R1, the refractive index of the first lens is n1, and R1-d1 * [1+ (n1-1) / n1] is δ, the following conditional expression: d2 <δ + fb2
It is characterized by satisfying.
The first surface of the first lens is the object side surface of the first lens. When the first lens is aspheric, the radius of curvature of the first surface of the first lens is the best fit R having an effective diameter.

According to the present invention, it is possible to reduce the size of the imaging optical system by using a lens having two lenses, and when the lens configuration satisfies the above conditional expression, light incident from outside the angle of view diverges on the image plane. It becomes light and can suppress the generation of ghost.
Specifically, as shown in FIG. 2, in the imaging optical system in which the first lens 11, the second lens 12, and the cover glass 13 are arranged in order from the object side to the image plane 14, d2 = δ + fb2. In this case, the light incident from the object side is emitted from the second lens 12 as parallel light L2. Further, when d2> δ + fb2, the light beam incident from the object side is emitted from the second lens 12 as the convergent light L3. Therefore, since the light beam incident on the lens is condensed on the image plane as parallel light L2 or convergent light L3, a ghost is likely to occur.

On the other hand, when the imaging optical system satisfies the above-described conditional expression d2 <δ + fb2, the light incident from the object side is emitted from the second lens 12 as the diverging light L1, and thus the generation of ghost can be suppressed. .
Further, since both the first lens and the second lens have positive power, the total length of the lens can be shortened, and the total length of the optical system can be further shortened. Furthermore, since the first lens has a meniscus shape, it is possible to reduce spherical aberration and increase light condensing performance.

The first lens is preferably an aspheric lens.
Thereby, various aberrations can be corrected according to the lens configuration, and an image with good performance can be obtained.

Furthermore, it is preferable to further have an aperture stop disposed between the first lens and the second lens.
In this way, by arranging the aperture stop between the first lens and the second lens, the total length of the imaging optical system can be increased as compared with the case where the aperture stop is disposed on the object side than the first lens or on the image plane side from the second lens. This can be further shortened, and as a result, the overall size can be reduced.

The image pickup apparatus according to the present invention includes the image pickup optical system and an image pickup unit that photoelectrically converts an image formed by the image pickup optical system and outputs an image signal.
According to the present invention, it is possible to provide an image pickup apparatus that can output a high-performance image that is small and suppresses the occurrence of ghosts by using the image pickup optical system as described above. Become.

According to the present invention, it is possible to reduce the size of the imaging optical system by using a lens having two lenses, and when the lens configuration satisfies the above conditional expression, light incident from outside the angle of view diverges on the image plane. It becomes light and can suppress the generation of ghost.
In addition, by using an imaging apparatus using the imaging optical system as described above, it is possible to provide an imaging apparatus that is small in size and capable of outputting a sharp image while suppressing the occurrence of ghosts.

1 is a side view showing a basic configuration of an imaging optical system according to an embodiment of the present invention. It is a figure explaining the conditional expression of the imaging optical system which concerns on embodiment of this invention. 1 is a side view illustrating a schematic configuration of an imaging apparatus according to an embodiment of the present invention. It is a figure regarding the imaging optical system of 1st Example, (A) is a side view which shows schematic structure of an imaging optical system, (B) is an aberrational figure. It is a figure regarding the imaging optical system of 2nd Example, (A) is a side view which shows schematic structure of an imaging optical system, (B) is an aberrational figure. It is a figure explaining the generation | occurrence | production principle of the ghost in the lens of 2 sheets structure.

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the drawings. However, the shape of the component described in this embodiment is not intended to limit the scope of the present invention, but merely an illustrative example.
FIG. 1 shows a basic configuration of an imaging optical system according to an embodiment of the present invention.
The imaging optical system has a two-lens lens composed of a first lens 11 and a second lens 12 in order from the object side.

The first lens 11 is a meniscus lens having a positive power with the convex surface facing the object side. The first lens 11 may be a spherical lens or an aspheric lens, but preferably at least one surface is an aspheric surface. This is because by using an aspherical surface, various aberrations can be corrected according to the lens configuration, and a sharp image can be obtained.
The second lens 12 is a lens having positive power. The second lens 12 may be any one of a biconvex shape, a plano-convex shape, and a meniscus shape. In particular, the second lens 12 is preferably a biconvex shape because the entire length of the lens can be shortened.

Referring to FIG. 2, the imaging optical system having the above-described configuration satisfies the following conditional expression.
d2 <δ + fb2
At this time, the distance between the first lens and the second lens: d2, the back focus when parallel light from the image plane side of the second lens is input: fb2, the thickness of the first lens: d1, the first lens first The radius of curvature of one surface is R1, the refractive index of the first lens is n1, and R1-d1 * [1+ (n1-1) / n1]: δ. The first surface of the first lens 11 is a surface located on the object side of the first lens 11. Further, when the first lens 11 is aspheric, the curvature radius R1 of the first surface of the first lens 11 is the best fit R having an effective diameter.

  In FIG. 2, when d2 = δ + fb2, the light beam incident from the object side is emitted from the second lens 12 as parallel light L2. Further, when d2> δ + fb2, the light beam incident from the object side is emitted from the second lens 12 as the convergent light L3. That is, since the light beam incident on the lens is condensed on the image plane 14 as parallel light L2 or convergent light L3, a ghost is likely to occur.

In contrast, according to the present embodiment, since the imaging optical system satisfies the conditional expression d2 <δ + fb2, the light incident from the object side is emitted from the second lens 12 as the diverging light L1, and thus the ghost. Can be suppressed.
Further, since both the first lens 11 and the second lens 12 have positive power, the total length of the lens can be shortened, and the total length of the optical system can be further shortened. Furthermore, since the first lens 11 has a meniscus shape, it is possible to reduce spherical aberration and increase light condensing performance.

In the present embodiment, in addition to the lens configuration described above, it is preferable to further include an aperture stop 15 disposed between the first lens 11 and the second lens 12.
In this manner, by arranging the aperture stop between the first lens 11 and the second lens 12, it is possible to take imaging optics as compared with the case where the aperture stop is arranged on the object side from the first lens 11 or on the image plane side from the second lens 12. The overall length of the system can be further shortened, and as a result, the overall size can be reduced.

Subsequently, an imaging apparatus according to an embodiment of the present invention will be described with reference to FIG.
This imaging apparatus mainly has an imaging optical system having the above-described configuration and imaging means.
Specifically, it has the 1st lens 11 and the 2nd lens 12 arrange | positioned on an optical axis, the image pick-up element 16 which is an imaging means, and the lens holder 17 holding the two lenses 11 and 12. FIG. Further, a cover glass 13 may be disposed between the second lens 12 and the image sensor 16. The image pickup device 16 photoelectrically converts an image formed by the image pickup optical system and outputs an image signal. For example, a CCD or CMOS can be used. The image plane 14 is formed on the light receiving surface of the imaging effective area 16a of the imaging device 16.
Furthermore, the aperture stop may be arrange | positioned between the 1st lens and the 2nd lens.

The imaging apparatus according to the present embodiment is an imaging apparatus using the imaging optical system as described above, thereby providing a small imaging apparatus capable of outputting a sharp image while suppressing the occurrence of ghosts. It becomes possible.
The imaging device described here is preferably used for various imaging devices such as an in-vehicle camera and a surveillance camera.

  Next, in each of the first lens, the second lens, and the cover glass of the imaging optical system having the above configuration, the radius of curvature R (mm), the distance (thickness) D (mm), the refractive index Nd, and the Abbe Each example carried out by specifically determining the number νd will be described. In the first embodiment, the surface marked with * represents an aspheric surface, and this aspheric surface is obtained by the following equation.

ここで、Ｚは非球面と光軸との交点を原点とした光軸方向の座標であり、ｈは前記原点を通り光軸に直交する方向の座標である。また、ｃは、ｃ＝１／ｒｉ（ｉ＝１〜４）であり、したがって、非球面は光軸近傍の曲率半径Ｒと、円錐定数Ｋと、高次の非球面係数ａｊ（ｊ＝２〜５）によって特定される。

Here, Z is the coordinate in the optical axis direction with the intersection of the aspherical surface and the optical axis as the origin, and h is the coordinate in the direction perpendicular to the optical axis through the origin. In addition, c is c = 1 / ri (i = 1 to 4). Therefore, the aspherical surface has a radius of curvature R near the optical axis, a conical constant K, and a higher-order aspherical coefficient aj (j = 2). ~ 5).

(First embodiment)
4A and 4B are diagrams relating to the imaging optical system of the first embodiment, FIG. 4A is a side view illustrating a schematic configuration of the imaging optical system, and FIG. 4B is an aberration diagram.
As shown in FIG. 4A, this imaging optical system is composed of two lenses, a first lens 11a and a second lens 12a, and further includes a cover glass 13a and an aperture stop 15a. The first lens 11a is a meniscus lens having positive power with the convex surface facing the object side, and the first surface and the second surface are aspherical surfaces. The second lens 12a has a positive power. In the figure, reference numeral 14a denotes an image plane of this imaging optical system.

  In the case of the first embodiment, (d2 =) 0.26 <0.096 + 0.768 (= δ + fb2), which satisfies the above conditional expression. Each aberration obtained here is shown in FIG. Spherical aberration, astigmatism, and distortion are shown in order from the left side of the figure. In the spherical aberration diagram, a, b, and c indicate three different wavelengths. In astigmatism, M indicates a meridional image plane, and S indicates a sagittal image plane.

(Second embodiment)
5A and 5B are diagrams relating to the imaging optical system of the second embodiment. FIG. 5A is a side view illustrating a schematic configuration of the imaging optical system, and FIG. 5B is an aberration diagram.
As shown in FIG. 5A, the imaging optical system is composed of two lenses, a first lens 11b and a second lens 12b, and further includes a cover glass 13b and an aperture stop 15b. The first lens 11a is a meniscus lens having a positive power with the convex surface facing the object side, and the first and second surfaces are spherical. The second lens 12b has a positive power. In the figure, reference numeral 14b denotes an image plane of this imaging optical system.

  In the case of the second embodiment, (d2 =) 0.265 <0.083 + 0.574 (= δ + fb2), which satisfies the above conditional expression. Each aberration obtained here is shown in FIG. Spherical aberration, astigmatism, and distortion are shown in order from the left side of the figure. In the spherical aberration diagram, a, b, and c indicate three different wavelengths. In astigmatism, M indicates a meridional image plane, and S indicates a sagittal image plane.

11, 11a, 11b First lens 12, 12a, 12b Second lens 13, 13a, 13b Cover glass 14, 14a, 14b Image plane 15, 15a, 15b Aperture stop 16 Image sensor 16a Imaging effective area 17 Lens holder

Claims (4)

From the object side,
A meniscus first lens having a positive power with a convex surface facing the object side;
A second lens having a second lens having positive power,
The distance between the first lens and the second lens is d2, the back focus when the parallel light from the image plane side of the second lens is input is fb2, the thickness of the first lens is d1, the first surface of the first lens is When the radius of curvature is R1, the refractive index of the first lens is n1, and R1-d1 * [1+ (n1-1) / n1] is δ, the following conditional expression: d2 <δ + fb2
An imaging optical system characterized by satisfying the above.   The imaging optical system according to claim 1, wherein the first lens is an aspheric lens.   The imaging optical system according to claim 1, further comprising an aperture stop disposed between the first lens and the second lens. The imaging optical system according to claim 1;
An imaging apparatus comprising: an imaging unit that photoelectrically converts an image formed by the imaging optical system and outputs an image signal.

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