JP3967465B2 - Objective lens - Google Patents

Description

【００３６】

【００３７】

【００３８】

【００３９】

ただし、ｒ₁，ｒ₂，・・・はレンズ各面の曲率半径、ｄ₁，ｄ₂，・・・は各レンズの肉厚およびレンズ間隔、ｎ₁，ｎ₂，・・・は各レンズの屈折率、ν₁，ν₂，・・・は各レンズのアッベ数である。
【００４０】
実施例１は、図１に示す通りの構成で、物体側より順に負の単レンズの第１レンズ群と平行平面板と、正の単レンズの第２レンズ群と明るさ絞りと、平行平面板を挟んで２枚の正の単レンズを配置した第２レンズ群と平行平面板とよりなる。
【００４１】
この実施例のレンズ系は、第３レンズ群の正の単レンズの曲面がいずれもその曲率中心が面よりも物体側にある球面にて形成されている。
【００４２】
又、この実施例は、第３レンズ群が２枚の正の単レンズよりなり、そのうちの像側の正の単レンズは平凸レンズである。又第１レンズ群の負の単レンズの物体側の面と第２レンズ群の正の単レンズの像側の面が平面である。
【００４３】
又、この実施例１のレンズ系は、条件（１）を満足し、これによって全長が５．１１０と短くなっている。
【００４４】
この実施例１は、像面湾曲、非点収差、コマ収差、倍率の色収差等の諸収差が実用上十分に補正されている。又平行平面板には、治療用レーザー光をカットコーティングがなされている。又第１面が平面であり、水切れがよい。
【００４５】
実施例２は図２に示す通りの構成で、第３レンズ群を構成する２枚の正の単レンズがいずれも物体側の面が平面である点で実施例１と相違するが他は実施例１と同じ構成である。
【００４６】
これら第３レンズ群のレンズの平面側にはレーザー光カットコーティングを容易に施すことができ、曲面に比べて入射する光線の角度のばらつきが少なく効果的である。各レンズはいずれも屈折率が１．７以上で、これにより、曲率半径が大であり、加工性がよく、薄肉であって全長が短い。
【００４７】
更に実施例２は、第２レンズ群と第３レンズ群のすべてのレンズの外径が等しく第１レンズ群の外径より小である。これにより保持枠の構造が簡単である。又第２レンズ群は、その物体側の凸面をレンズ受け部にて受けるようにでき、これにより、ピントや画角に最も影響する面の偏芯を抑えることができる。
【００４８】
実施例３および実施例４は図３，図４に示す通りの構成のレンズ系で、実施例２と同様の構成である。
【００４９】
又実施例３は、低融点硝材（住田光学社製ＳＦＬＤ６０、ｎ＝１．８０６４５，ν＝２４．４，転移点５０３℃）を用いてモールドによりレンズｎ₂を成形しえるようにした例であり、実施例４は赤外線吸収素材（ＨＯＹＡ社製Ｃ５０００、ｎ＝１．５１４、ν＝７５）からなるレンズｎ₄を用いた例で、したがってこのレンズの屈折率は１．７以下である。
【００５０】
実施例５は、図５に示す通りで、レンズ系の構成は、第３レンズ群の像側に平行平面板を１枚多く配置した点で実施例２〜４と相違するが、その他の点ではこれら実施例と類似の構成である。
【００５１】
又、図５は実施例５の対物レンズをレンズ枠に取り付けた状態を示している。この実施例５は、負の単レンズＬ１の第１レンズ群と、正の単レンズＬ２の第２レンズ群と正の単レンズＬ３と正の単レンズＬ４よりなる第３レンズ群の４枚のレンズよりなる。これらレンズを保持する保持枠１は、ほぼ筒状で内径の大きい大径部２と内径が小さい小径部３と大径部２と小径部３との中間の内径を有する中径部４とを有し、大径部２と小径部３との境に内側へ突出するレンズ受け部５を有する構成である。
【００５２】
このような、枠１の大径部２には、板状のフレア絞り６と負の単レンズＬ１が接着剤により固定される。又小径部３には正の単レンズＬ２と、間隔環７と、正の単レンズＬ３と、間隔環９と正の単レンズＬ４とが順次挿入される。つまり、小径部３内に平凸レンズＬ２が凸面を先方に向け挿入され、レンズ受け部５が挿入されて当てつけることにより位置決めされる。続いて間隔環７と板状明るさ絞り８とレンズＬ３と間隔環９と板状フレア絞り１０とが順次小径部３内に挿入され、最後にレンズＬ４が挿入される。更に間隔環１１が小径部３の端より一部中径部４の側に突出するように挿入され、板状フレア絞り１２とフィルター１３とが間隔環１１に押し付けられるようにして中径部４内に挿入され、小径部３と中径部４とに挿入された各要素がレンズ受け部５とフィルター１３との間で位置決め保持される。図示するように、フィルター１３は、その一部が保持枠１より外部に露出しており、このフィルター１３の露出部分の側面と保持枠１の中径部４の端面との間で接着剤１４により接着固定される。
【００５３】
このようにして一体化された対物レンズユニットＵは外枠により撮像素子と一体化される。つまり、外枠１５は、その一端部（像側の端部）の内径が他と比べて若干大になっており、ここにフレア絞り１６を有する第２のフィルター１７と、この第２のフィルター１７に接合されたカバーガラス１８が固着されている。又カバーガラス１８は、図示しない固体撮像素子の撮像面を保護するためのものであり、撮像素子に対し固定された位置関係にある。
【００５４】
このように前記各要素を固定した外枠１５の反対側（図面左側）より対物レンズユニットＵを挿入して、矢印Ａ方向である光軸に沿った方向の位置調整を行ない、撮像面と対物レンズユニットＵとの間隔調整、いわゆるピント調整を行なった上で、接着剤１９により保持枠１と外枠１５とを固定する。このようにして対物レンズから撮像素子までを一体化した撮像ユニットが完成する。
【００５５】
この実施例５は、レンズＬ２からレンズＬ４まで（第２レンズ群と第３レンズ群）のレンズの外径を等しくしてあり、これにより保持枠１の小径部３の内面を単純な筒状にすればよいので、枠の加工が極めて簡単である。
【００５６】
また、対物レンズの最も像側の要素（レンズＬ４）の外径φ４と、その射出側に設けられた複数の平行平面板の対物レンズに近い側から順に見た二つの平行平面板（フィルター１３、１７）の外径φ５、φ６が条件（２）を満足するように構成されている。この条件（２）を満足することにより、レンズ組立の際に、これら要素を保持枠あるいは外枠のような保持部材に取り付けることが容易である。
【００５７】
また、この実施例では、明るさ絞りをレンズ系の中央付近に設けてあり、そのために主光線が像面に近づくにつれて高くなり、光線のけられを防ぐためにも上記条件（２）を満足するように構成して要素の外径が徐々に大になるようにすることが望ましい。
【００５８】
又、第２レンズ群のレンズＬ２は、偏芯や前後の位置ずれ等による光学性能の劣化に大きく影響するレンズであるため、このレンズＬ２を保持枠１に設けたレンズ受け部５にその凸面を当てつけるようにして保持することにより、後方（像側）よりの押圧力によりレンズＬ２のセンタリングを容易に行ない得、しかも受け部５が保持枠１と一体に形成され保持枠の一部をなすため間隔環を介して押し付け保持する方法に比べて、レンズ保持の安定性が高く、長期にわたって位置ずれが起こりにくい。
【００５９】
以上述べたように、本発明の対物レンズは、特許請求の範囲に記載するレンズ系のほか、次の各項に記載する構成のものもその目的を達成し得るものである。
【００６０】
（１） 特許請求の範囲の請求項３に記載するレンズ系で、第１レンズ群の負の単レンズの物体側の面が平面であることを特徴とする対物レンズ。
【００６１】
（２） 特許請求の範囲の請求項１、２又は３あるいは前記の（１）の項に記載するレンズ系で、下記条件（１）を満足することを特徴とする対物レンズ。
（１） ０．７＜Ｄ（Ｔ）／ｆ＜３．５
【００６２】
（３） 特許請求の範囲の請求項１、２又は３あるいは前記の（１）又は（２）の項に記載するレンズ系で、第１、第２、第３レンズ群のすべてのレンズの屈折率が１．７以上であることを特徴とする対物レンズ。
【００６３】
（４） 特許請求の範囲の請求項１、２又は３あるいは前記の（１）、（２）又は（３）の項に記載するレンズ系で、第２レンズ群と第３レンズ群のすべてのレンズの外径が等しく、これらレンズの外径が第１レンズ群の外径よりも小であることを特徴とする対物レンズ。
【００６４】
（５） 特許請求の範囲の請求項１、２又は３あるいは前記の（１）、（２）、（３）又は（４）の項に記載するレンズ系で、第２レンズ群が物体側に凸のレンズで該レンズの凸の面を保持枠に一体に形成されたレンズ受け部にて受けるようにして保持枠に固定することを特徴とする対物レンズ。
【００６５】
（６） 特許請求の範囲の請求項１、２又は３あるいは前記の（１）、（２）、（３）、（４）又は（５）の項に記載するレンズ系で、レンズ系により形成される像を電気信号に変換する固体撮像素子と、レンズ系と固体撮像素子との間に配置された少なくとも２枚のほぼ透明な平行平面板とを備え、前記平行平面板のうち最も物体側に位置する平行平面板がレンズ系と同一の枠に固定され、下記条件（２）を満足することを特徴とする対物レンズ。
（２） φ（４）＜φ（５）＜φ（６）
【００６６】
【発明の効果】
本発明は、単レンズのみの簡単な構成で、しかも実用上十分な収差補正がなされた小型で全長の短い対物レンズである。
【図面の簡単な説明】
【図１】 本発明の実施例１の断面図
【図２】 本発明の実施例２の断面図
【図３】 本発明の実施例３の断面図
【図４】 本発明の実施例４の断面図
【図５】 本発明の実施例５の断面図
【図６】 従来の対物レンズの断面図
【図７】 他の従来の対物レンズの断面図
【図８】 他の従来の対物レンズの断面図
【図９】 他の従来の対物レンズの断面図[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an objective lens suitable for an endoscope, digital camera, surveillance television camera, etc., which is a small lens system having a small number of lenses.
[0002]
[Prior art]
In recent years, small-diameter endoscopes for observing and treating thin lumens such as bronchi have been put into practical use. The objective lens used in these small-diameter endoscopes has a sufficiently corrected aberration, and the endoscope insertion section is made thinner and the distal end of the endoscope insertion section is hard to improve operability. Shortening of the part is desired.
[0003]
In addition, when transmitting an image formed by an objective lens with an optical fiber in a fiber scope, when transmitting an image of an objective lens with a relay lens in a rigid scope, or on the light receiving surface of a solid-state image sensor in a video scope. In the case of converting the image formed in (1) to an electric signal, it is desirable that the light beam from the objective lens is incident substantially perpendicular to the light receiving surface. That is, it is desirable that the lens system is telecentric. In image transmission systems and the like, it is necessary to prevent loss of light quantity during image transmission. When receiving light with a solid-state image sensor, it is necessary to prevent deterioration in image quality due to the occurrence of luminance shading and color shading. It is.
[0004]
Further, a lens system used in a digital camera or a surveillance television camera is also desired to be reduced in size and weight, and to have a telecentric configuration as described above.
[0005]
A lens system described in Japanese Patent Application Laid-Open No. 5-10747 is known as a conventional example of a small-sized endoscope objective lens that meets the above demand. For example, as shown in FIG. 6, the lens system described in Example 3 in the above publication includes, in order from the object side, a first lens composed of a negative single lens, a second lens composed of a positive single lens, and brightness. This is a lens system including a diaphragm and three single lenses including a third lens including a positive single lens.
[0006]
In this conventional example, the chromatic aberration of magnification is corrected, but the power of the concave lens, which is the first lens, is strong, so that the field curvature increases, and two convex lenses (second lens and third lens) are arranged thereafter. ) Is difficult to correct sufficiently. Also, if the total length of the objective lens is to be shortened in order to shorten the distal end hard portion of the endoscope insertion portion, a telecentric configuration is required with only one convex lens (third lens) after the aperture stop. Is difficult.
[0007]
As another conventional example, a lens system described in JP-A-7-84179 is known. This conventional example has a configuration as shown in FIG. 7, and in order from the object side is a first lens made of a negative single lens, an aperture stop, a second lens made of a positive single lens, and a positive single lens. The third lens system is composed of three lenses, and is a lens system that is more easily telecentric than the conventional example. However, the curvature of field is not sufficiently corrected, and the chromatic aberration of magnification is not sufficiently corrected.
[0008]
As another conventional example, an objective lens described in Japanese Patent Publication No. 8-27429 is known. In this conventional example, as shown in FIG. 8, in order from the object side, a first lens composed of a negative single lens, a second lens composed of a positive single lens, an aperture stop, and a cemented lens as a whole are positive. And a third lens.
[0009]
In this conventional lens system, chromatic aberration of magnification and curvature of field are well corrected, and a telecentric configuration is also possible.
[0010]
However, since a cemented lens is used as the third lens, it is not preferable because a process such as centering and bonding is required for processing as compared with a single lens, and costs increase. In addition, the cemented lens must ensure a certain edge thickness of the positive lens and a medium thickness of the negative lens, which are processed, and it is difficult to shorten the overall length of the objective lens.
[0011]
As another conventional example, a lens system described in JP-A-9-61710 is known. This conventional example, as shown in FIG. 9, is composed of a first lens composed of a negative single lens, a second lens composed of a positive single lens, an aperture stop, and a positive single lens in order from the object side. This is a lens system composed of four lenses composed of a third lens and a fourth lens composed of a single positive lens.
[0012]
In this conventional example, various aberrations such as chromatic aberration of magnification, curvature of field, and astigmatism are well corrected, and a practically sufficient telecentric lens system can be obtained.
[0013]
However, in this conventional example, the fourth lens is a convex surface on the object side, and the back focus is shortened. In order to shorten the distal end hard portion of the endoscope insertion portion, it is necessary to shorten the entire length of the lens system, but in order to adjust the focus of the lens system, a long back focus is desirable.
[0014]
In the lens system of this conventional example, as described above, the fourth lens is a convex surface on the object side. When this lens is bonded to the cover glass of an image sensor such as a CCD, the adhesive used is assemblability. To UV curable materials are used. For this reason, in the case of the configuration as in the conventional example, the number of times the ultraviolet ray is applied to the image sensor increases, and there is a high possibility that the color correction filter arranged on the image sensor is adversely affected.
[0015]
[Problems to be solved by the invention]
The present invention provides an objective lens that can be made telecentric, shortened in overall length and reduced in outer diameter for miniaturization, and has various aberrations corrected satisfactorily.
[0016]
[Means for Solving the Problems]
The objective lens according to the present invention includes, in order from the object side, a first lens group including one negative single lens, a second lens group including one positive single lens, an aperture stop, and two single lenses. Each of the surfaces having the curvature of all the single lenses constituting the positive third lens group is located on the object side with respect to the surface. The following condition (1) is satisfied.
(1) 0.7 <D (T) / f <3.5
Here, D (T) is the sum of the center thicknesses of all the lenses having a curved surface in the objective lens, and f is the focal length of the entire objective lens system.
[0017]
The objective lens of the present invention has a lens system in which a third lens group, which is composed of a plurality of single lenses (two or more single lenses) and is a positive lens group as a whole, is disposed between the aperture stop and the image plane. Is configured to have sufficient telecentricity, and the field curvature can be corrected well.
[0018]
In addition, a lens system having a long back focus while shortening the overall length of the lens system so that all the single lenses constituting the third lens group are located on the object side with respect to the surface having the curvature. I made it. In the lens system of the present invention, the third lens group closer to the image side than the aperture stop is composed of a plurality of single lenses, so that spherical aberration, coma, astigmatism, curvature of field, chromatic aberration of magnification, etc. Various aberrations were corrected sufficiently satisfactorily.
[0019]
That is, the objective lens according to the present invention has a short overall length, a long back focus, sufficient telecentricity, and various aberrations can be corrected sufficiently satisfactorily by adopting the lens system having the above-described configuration. .
[0020]
In the objective lens of the present invention, if the third lens group is constituted by two single lenses, the above-mentioned various aberrations can be corrected satisfactorily. Therefore, the object of the present invention is achieved with a small lens system having a small number of lenses. Can be made into a possible lens system.
[0021]
In the lens system of the present invention, if the single lens constituting the third lens group is a plano-convex lens, the processing of each single lens is facilitated and the assemblability is improved. Furthermore, when a coating that cuts light of a specific wavelength, such as an interference type YAG laser cut coat, is applied to the plane of a single lens, the coating is easy. Further, since the angle of the laser beam or the like incident on the coat can be made smaller than that when incident on the spherical surface, the laser beam can be cut more effectively.
[0022]
Further, if there is a flat surface in the lens system, filters such as an infrared cut filter can be arranged without providing a useless space. In addition, even when a filter made of a soft and easily damaged material such as C5000 (product number, catalog number) manufactured by Hoya Co., Ltd. is placed in contact with the lens, it can be disposed of on the flat side of the lens. Etc., and the filter can be prevented from being damaged.
[0023]
In the objective lens of the present invention, it is desirable to have the following configuration. That is, in order from the object side, a first lens group composed of one negative single lens, a second lens group composed of one positive single lens, an aperture stop, and a third lens composed of two positive single lenses. The lens system includes a lens group, and the object side surface of the first lens group is a flat surface. As a result, the workability of the first lens group is improved, and the first surface is flat, so that the water drains off.
[0024]
Further, it is preferable that the third lens group is constituted by two single lenses, and the image side surface of the second lens group and the object side surface of the two single lenses of the third lens group are both flat. Accordingly, it is preferable to arrange a filter between the lenses of the second lens group and the third lens because it can be easily arranged without taking up a large space.
[0025]
Further, according to the present invention, the lens systems having the above-described configurations, that is, in order from the object side, a first lens group including one negative single lens, a second lens group including one positive single lens, and a brightness a diaphragm is, is constituted by the third lens group is a positive lens group composed of a plurality of single lenses, (b) the center of curvature of the surface on which all of the single lens of the third lens group has a curvature of the surface A lens characterized by being located on the object side, (b) the third lens group is composed of two single lenses, and the single lens has a curvature center on the object side of the surface with curvature. (C) the third lens group is composed of two single lenses, and the object-side surfaces of the two single lenses are both flat; The object side surface of one lens unit is a flat surface, the image side surface of the second lens unit and 2 of the third lens unit. In lens system surface on the object side of the single lens is characterized in that the plane of, it is desirable to satisfy the following condition (1).
(1) 0.7 <D (T) / f <3.5
Here, D (T) is the sum of the center thicknesses of all the lenses having a curved surface in the objective lens, and f is the focal length of the entire objective lens system.
[0026]
In the condition (1), the lens system can be shortened when D (T) / f is small. The upper limit of condition (1) is a value that is practically acceptable for endoscopes and the like. If the upper limit of 3.5 is exceeded, the total length of the lens system cannot be shortened. On the other hand, when the lower limit of 0.7 is exceeded, the center thickness of the lens becomes small and the processability of the lens deteriorates.
[0027]
If the upper limit of condition (1) is set to 2 instead of 3.5 to satisfy the following condition (1-1), it is more preferable for shortening the lens system.
(1-1) 0.7 <D (T) / f <2
[0028]
In the lens system of the present invention, it is desirable that the refractive index n of the optical element (lens) constituting the lens system excluding optical components made of infrared absorbing material is 1.7 or more.
[0029]
If the refractive index n of the lens is 1.7 or more, the curvature of the lens can be relaxed (the radius of curvature can be increased), the lens can be easily processed, and the lens can be made thin. You can get none. When the lens is formed of an infrared absorbing material, for example, when C5000 or HA15 (product number: catalog number) manufactured by Hoya Co., Ltd. is used, the refractive index of these infrared absorbing materials is around 1.5. is there. Therefore, when a lens for such purpose is introduced, the refractive index of this lens is 1.7 or less.
[0030]
In the objective lens of the present invention, it is desirable that the outer diameters of the lenses constituting the lens system other than the negative single lens that is the first lens group are equal and smaller than the first lens group. In this way, the internal structure of the frame that accommodates the lens can be simplified, and the workability of the frame and the assembly of the lens can be improved.
[0031]
In the lens system of the present invention, when the positive second lens group is a single lens convex on the object side, decentering or the like can be suppressed by receiving the convex surface on the object side of the convex single lens with a frame. .
[0032]
This positive second lens group is a lens having a large influence on the optical performance (influence on the focus, the angle of view, etc.) due to the positional deviation and decentration. If this lens is a lens having a convex surface on the object side, and this convex surface is received by a lens receiving portion formed integrally with the holding frame, the eccentricity of this lens can be suppressed, and deterioration of optical performance can be prevented. .
[0033]
In addition, when an image obtained by the lens system of the present invention is received by a solid-state image pickup device such as a CCD and exchanged with an electric signal, two or more substantially transparent plane-parallel plates are disposed between the objective lens and the solid-state image pickup device. If the parallel plane plate closest to the object is fixed to the same frame as the lens system, the interference between the final curved surface of the objective lens and the CCD cover glass can be reduced when adjusting the focus of the lens system. Can be prevented. In this case, it is desirable that the outer diameter of the plane parallel plate satisfies the following condition (2).
(2) φ (4) <φ (5) <φ (6)
Where φ (4) is the outer diameter of the lens closest to the image side of the third lens group, φ (5) is the outer diameter of the parallel plane plate closest to the object among the plurality of parallel plane plates, and φ (6) is This is the diameter of the parallel plane plate located second from the object side of the parallel plane plates.
[0034]
DETAILED DESCRIPTION OF THE INVENTION
The following embodiments of the objective lens of the present invention will be described based on examples having the following data.
[0035]

[0036]

[0037]

[0038]

[0039]

Where r ₁ , r ₂ ,... Are the radii of curvature of the lens surfaces, d ₁ , d ₂ ,... Are the thickness of each lens and the lens spacing, and n ₁ , n ₂ ,. , Ν ₁ , ν ₂ ,... Are Abbe numbers of the respective lenses.
[0040]
Example 1 has the configuration shown in FIG. 1, and in order from the object side is a negative single lens first lens group, a plane parallel plate, a positive single lens second lens group, an aperture stop, and a parallel flat plate. It consists of a second lens group in which two positive single lenses are arranged across a face plate and a plane parallel plate.
[0041]
In the lens system of this example, the curved surface of each positive single lens in the third lens group is formed as a spherical surface whose center of curvature is on the object side of the surface.
[0042]
In this embodiment, the third lens group includes two positive single lenses, and the positive single lens on the image side is a plano-convex lens. The object side surface of the negative single lens of the first lens group and the image side surface of the positive single lens of the second lens group are flat surfaces.
[0043]
In addition, the lens system of Example 1 satisfies the condition (1), and the overall length is shortened to 5.110.
[0044]
In Example 1, various aberrations such as field curvature, astigmatism, coma aberration, and chromatic aberration of magnification are sufficiently corrected in practice. The parallel flat plate is cut and coated with a therapeutic laser beam. Further, the first surface is a flat surface, and the water runs out.
[0045]
The second embodiment is configured as shown in FIG. 2 and differs from the first embodiment in that both of the two positive single lenses constituting the third lens group have a plane on the object side. The configuration is the same as in Example 1.
[0046]
A laser light cut coating can be easily applied to the plane side of the lenses of the third lens group, and the variation in the angle of the incident light beam is small and effective compared to the curved surface. Each lens has a refractive index of 1.7 or more, thereby having a large radius of curvature, good workability, thin wall, and short overall length.
[0047]
Further, in Example 2, the outer diameters of all the lenses of the second lens group and the third lens group are equal and smaller than the outer diameter of the first lens group. Thereby, the structure of the holding frame is simple. In addition, the second lens group can receive the convex surface on the object side by the lens receiving portion, thereby suppressing the eccentricity of the surface that most affects the focus and the angle of view.
[0048]
Example 3 and Example 4 are lens systems configured as shown in FIGS. 3 and 4 and have the same configuration as Example 2. FIG.
[0049]
Example 3 is an example in which a lens n ₂ can be formed by molding using a low melting glass material (SFLD60 manufactured by Sumita Optical Co., Ltd., n = 1.86045, ν = 24.4, transition point 503 ° C.). In Example 4, a lens n ₄ made of an infrared absorbing material (C5000 manufactured by HOYA, n = 1.514, ν = 75) is used. Therefore, the refractive index of this lens is 1.7 or less.
[0050]
Example 5 is as shown in FIG. 5, and the configuration of the lens system is different from Examples 2 to 4 in that one parallel plane plate is arranged on the image side of the third lens group. Then, it is a structure similar to these Examples.
[0051]
FIG. 5 shows a state in which the objective lens of Example 5 is attached to the lens frame. In Example 5, the first lens group of the negative single lens L1, the second lens group of the positive single lens L2, the third lens group including the positive single lens L3 and the positive single lens L4 are used. It consists of a lens. A holding frame 1 for holding these lenses has a large-diameter portion 2 having a substantially cylindrical shape and a large inner diameter, a small-diameter portion 3 having a small inner diameter, and a medium-diameter portion 4 having an inner diameter intermediate between the large-diameter portion 2 and the small-diameter portion 3. And having a lens receiving portion 5 protruding inward at the boundary between the large diameter portion 2 and the small diameter portion 3.
[0052]
A plate-like flare stop 6 and a negative single lens L1 are fixed to the large-diameter portion 2 of the frame 1 with an adhesive. Further, a positive single lens L2, a spacing ring 7, a positive single lens L3, a spacing ring 9 and a positive single lens L4 are sequentially inserted into the small diameter portion 3. That is, the plano-convex lens L2 is inserted into the small diameter portion 3 with the convex surface facing forward, and the lens receiving portion 5 is inserted and positioned. Subsequently, the interval ring 7, the plate-like brightness stop 8, the lens L3, the interval ring 9, and the plate-like flare stop 10 are sequentially inserted into the small diameter portion 3, and finally the lens L4 is inserted. Further, the spacing ring 11 is inserted so as to partially protrude from the end of the small-diameter portion 3 toward the middle-diameter portion 4, so that the plate-like flare stop 12 and the filter 13 are pressed against the spacing ring 11. Each element inserted into the small diameter portion 3 and the medium diameter portion 4 is positioned and held between the lens receiving portion 5 and the filter 13. As shown in the drawing, a part of the filter 13 is exposed to the outside from the holding frame 1, and an adhesive 14 is provided between the side surface of the exposed portion of the filter 13 and the end surface of the medium diameter portion 4 of the holding frame 1. Is fixed by adhesion.
[0053]
The objective lens unit U integrated in this way is integrated with the image sensor by the outer frame. That is, the outer frame 15 has a slightly larger inner diameter at one end (image-side end) than the other, and a second filter 17 having a flare stop 16 here, and the second filter. A cover glass 18 joined to 17 is fixed. The cover glass 18 is for protecting the imaging surface of a solid-state imaging device (not shown), and has a fixed positional relationship with the imaging device.
[0054]
In this way, the objective lens unit U is inserted from the opposite side (left side of the drawing) of the outer frame 15 to which the respective elements are fixed, and the position in the direction along the optical axis, which is the direction of the arrow A, is adjusted. After adjusting the distance from the lens unit U, so-called focus adjustment, the holding frame 1 and the outer frame 15 are fixed by the adhesive 19. In this way, an imaging unit in which the objective lens and the imaging element are integrated is completed.
[0055]
In the fifth embodiment, the outer diameters of the lenses from the lens L2 to the lens L4 (second lens group and third lens group) are made equal, and thereby the inner surface of the small-diameter portion 3 of the holding frame 1 is a simple cylindrical shape. Therefore, the processing of the frame is extremely simple.
[0056]
Also, the outer diameter φ4 of the most image side element (lens L4) of the objective lens and two parallel plane plates (filter 13) viewed in order from the side closer to the objective lens of the plurality of plane parallel plates provided on the exit side. 17), the outer diameters φ5 and φ6 satisfy the condition (2). By satisfying this condition (2), it is easy to attach these elements to a holding member such as a holding frame or an outer frame during lens assembly.
[0057]
In this embodiment, the aperture stop is provided near the center of the lens system. For this reason, the principal ray increases as it approaches the image plane, and the above condition (2) is satisfied in order to prevent the ray from being scattered. It is desirable to configure so that the outer diameter of the element gradually increases.
[0058]
Further, the lens L2 of the second lens group is a lens that greatly affects the deterioration of the optical performance due to decentering, front / rear positional deviation, and the like, so that the convex surface is provided on the lens receiving portion 5 provided on the holding frame 1. , The lens L2 can be easily centered by a pressing force from the rear (image side), and the receiving portion 5 is formed integrally with the holding frame 1 and a part of the holding frame is formed. Therefore, as compared with the method of pressing and holding through the interval ring, the lens holding stability is high, and the positional deviation is less likely to occur over a long period of time.
[0059]
As described above, the objective lens according to the present invention can achieve its object in addition to the lens system described in the claims, as well as the configurations described in the following items.
[0060]
(1) An objective lens according to claim 3, wherein the object side surface of the negative single lens of the first lens group is a flat surface.
[0061]
(2) An objective lens characterized by satisfying the following condition (1) in the lens system described in claim 1, 2, 3 or claim (1).
(1) 0.7 <D (T) / f <3.5
[0062]
(3) Refraction of all lenses of the first, second, and third lens groups in the lens system according to claim 1, 2, or 3, or (1) or (2) above. An objective lens having a ratio of 1.7 or more.
[0063]
(4) In the lens system according to claim 1, 2 or 3 of the claims or the item (1), (2) or (3), all of the second lens group and the third lens group An objective lens, wherein the outer diameters of the lenses are equal, and the outer diameters of these lenses are smaller than the outer diameter of the first lens group.
[0064]
(5) In the lens system according to claim 1, 2 or 3 of the claims or the item (1), (2), (3) or (4), the second lens group is located on the object side. An objective lens, wherein a convex lens is fixed to a holding frame so that a convex surface of the lens is received by a lens receiving portion formed integrally with the holding frame.
[0065]
(6) The lens system according to claim 1, 2 or 3 of the claims or the above (1), (2), (3), (4) or (5). A solid-state imaging device for converting the image to be converted into an electrical signal, and at least two substantially transparent parallel plane plates disposed between the lens system and the solid-state imaging device, the most object side of the parallel plane plates An objective lens, characterized in that the plane parallel plate located at is fixed to the same frame as the lens system and satisfies the following condition (2).
(2) φ (4) <φ (5) <φ (6)
[0066]
【The invention's effect】
The present invention is a small objective lens that has a simple structure with only a single lens and has a practically sufficient aberration correction and a short overall length.
[Brief description of the drawings]
1 is a cross-sectional view of a first embodiment of the present invention. FIG. 2 is a cross-sectional view of a second embodiment of the present invention. FIG. 3 is a cross-sectional view of a third embodiment of the present invention. Cross-sectional view [FIG. 5] Cross-sectional view of Example 5 of the present invention [FIG. 6] Cross-sectional view of a conventional objective lens [FIG. 7] Cross-sectional view of another conventional objective lens [FIG. Cross section [Fig. 9] Cross section of another conventional objective lens

Claims (2)

In order from the object side, the first lens group including one negative single lens, the second lens group including one positive single lens, an aperture stop, and two single lenses as a whole are positive first. Each surface having the curvature of all the single lenses constituting the positive third lens group is composed of three lens groups, and the center of curvature is located on the object side of the surface, and satisfies the following condition (1) An objective lens characterized by that.
(1) 0.7 <D (T) / f <3.5
Here, D (T) is the sum of the center thicknesses of all the lenses having a curved surface in the objective lens, and f is the focal length of the entire objective lens system.     2. The objective lens according to claim 1, wherein both object-side surfaces of the two single lenses of the third lens group are flat surfaces.

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