JP4197897B2 - Imaging optical system and endoscope using the same - Google Patents

Description

【００７８】
実施例２

【００７９】
実施例３

【００８０】
実施例４

【００８１】
実施例５

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空気間隔、ｎ₁，ｎ₂、・・・は各レンズのｄ線に対する屈折率、ν₁，ν₂、・・・は各レンズのｄ線に対するアッベ数である。尚、ｒ₀は物体面、ｄ₀は物体面からカバーガラスまでの距離である。
【００８２】
実施例１の撮影光学系は、図１に示すように、カバーガラスＣ１と、負の屈折力を持つ前群ＧＦと明るさ絞りＳと正の屈折力を持つ後群ＧＲとよりなる対物レンズと、フィルターＦ１、Ｆ２と、撮像素子カバーガラスＣ２と撮像素子封止ガラスＣ３とよりなり、前群ＧＦが両凹レンズ（ｒ₃〜ｒ₄）よりなり、後群ＧＲが正レンズ（ｒ₆〜ｒ₇）と正レンズと負レンズとを接合した正の接合レンズ（ｒ₉〜ｒ₁₁）とよりなる。尚ＦＳはフレアー絞りである。
【００８３】
また実施例２〜５は夫々図２〜図５に記載する通りの構成で、実施例１と実質上同じレンズ構成である。
【００８４】
図６〜図１０は夫々前記実施例１〜５の収差曲線図である。
【００８５】
これら実施例１〜５の光学系における結像光、ゴースト光について述べる。
【００８６】
実施例１の撮影光学系において、正規光の結像光路、ゴースト光の結像光路を図１２、１３に示す。
【００８７】
これら図において、図１２は物点から射出した光線が、本発明の撮影光学系（実施例１）に入射角１０°にて入射した時の正規の結像光路を示す。
【００８８】
この場合の近軸結像位置は、面ｒ₁₉より像側に０．０５７の距離である。
【００８９】
また、図１３は、物点から射出して実施例１の撮影光学系に入射角１０°にて入射するゴースト光の結像光路を示す。
【００９０】
このゴースト光の近軸の結像位置は、第１９面（ｒ₁₉）より物体側０．９６５の位置である。つまり、図１３のｒ₁₄とｒ₁₅の間の位置であり、フィルターＦ中である。
【００９１】
上記のようにゴースト光の結像位置Ｑ’は、正規光の結像位置Ｐ’より物体側に１．０２２ずれている。また、画面上の結像位置（光軸と垂直な方向の結像位置）に関しては、通常光の結層位置よりも画面の外側に（光軸から離れる方向に）ずれている。
【００９２】
この結果、ゴーストがボケて明瞭ではなくなる。しかも輝度の高い被写体が画面周辺部にあれば、ゴースト画面外に外れ観察されなくなる。
【００９３】
図１８は、画面上の通常光と、ゴースト光の位置関係を示す。
【００９４】
この図１８において、Ｒ１は通常光（結像光）の画面中心近くの像、ＲＧ１は通常光Ｒ１によるゴースト、Ｒ２は通常光の画面周辺部の像、ＲＧ２は通常光Ｒ２によるゴーストである。
【００９５】
この図に示すように、ゴースト光ＲＧ１は、結像位置が画面から光軸方向にずれているためボケている。また、ゴースト光ＲＧ２は、ボケていると共に画面外に外れている。
【００９６】
以上は、実施例１で、オートクレーブを用いた装置における光学系である。
【００９７】
また、オートクレーブを用いていない装置の場合、撮影光学系は図に示す構成でもよい。
【００９８】
図１４はカバーガラスを用いていない撮影光学系であり、撮像素子のカバーガラスＣ２のガラス材料としてＳ−ＢＳＬ７（株式会社オハラ製）を用いたものである。
【００９９】
また、図１５は、物体側のカバーガラスＣ１の材料として、Ｓ−ＢＳＬ７を用いたものである。
【０１００】
このように、オートクレーブを用いない装置に使用する撮影光学系は、カバーガラスの材料をサファイヤにする必要はない。そのため、カバーガラスの材料としてＢ−ＢＳＬ７を用いてコストを低減させることができる。
【０１０１】
また、すべての実施例の光学系において、鉛や砒素を含まない硝材にて構成することができる。
【０１０２】
例えば、実施例１においては、物体側から順に、カバーガラスＣ１がサファイヤ、第１レンズ（ｒ₃〜ｒ₄）及び第２レンズ（ｒ₆〜ｒ₇）がＳ−ＬＡＨ５８（株式会社オハラ製）、第３レンズ（ｒ₉〜ｒ₁₀）がＳ−ＮＰＨ２（株式会社オハラ製）、フィルターＦ１が白板またはＳ−ＢＳＬ７（株式会社オハラ製）、フィルターＦ２がＣＭ５０００（ＨＯＹＡ株式会社製）、カバーガラスＣ２がサファイヤ、撮像素子封止ガラスＣ３がＳ−ＢＳＬ７（株式会社オハラ製）である。
【０１０３】
したがって、オートクレーブ滅菌への耐性があって、ゴーストの目立たない構成であり、小型で、個々のレンズの加工性がよく、また有害な鉛、砒素を含まず環境にとっても好ましい構成の撮影光学系である。また、この実施例の撮影光学系は、耐用年数が過ぎた後に回収し、これを分解廃棄する時も、廃棄コストを低減し得る。
【０１０４】
以上、実施例１について述べたが、実施例２〜４の撮影光学系も、実施例１と類似のレンズ構成であり、使用する硝材も同様の硝材である。
【０１０５】
したがって、実施例１にて述べたと同様の特徴を有している。
【０１０６】
実施例５は、図５に示す通りの構成である。この実施例５は、他の実施例とレンズ構成が類似する光学系である。ｆ／ｒ₃の値が条件（１）の上限に近い値であるが、本実施例においてもゴーストを改善できる。つまり、ゴースト光の結像位置Ｑ’が正規光の結像位置Ｐ’よりも物体側にずれている。
【０１０７】
図１６は、この実施例５の光学系において、物点から射出して、入射角２０°にて撮影光学系に入射する正規の結像光路を示す。この光学系の近軸の結像位置は、第１９面（ｒ₁₉）から像側に０．０８２の距離にある。
【０１０８】
また図１７は、この実施例５において、物点から射出して入射角２０°にて撮影光学系に入射するゴースト光の結像光路を示す。
【０１０９】
この実施例５は、ゴースト光の近軸の結像位置が第１９面（ｒ₁₉）より像側に０．０２５の距離である。
【０１１０】
この実施例５は、ゴースト光の結像位置Ｑ’が正規光の結像位置より物体側に０．０５７ずれている。
【０１１１】
本発明は以上詳細に説明した通りであり、特許請求の範囲に記載した発明のほか、次の各項に記載する発明もその目的を達成し得る。
【０１１２】
（１） 特許請求の範囲の請求項１に記載する光学系で、下記条件（１）を満足することを特徴とする撮影光学系。
（１） −２＜ｆ／ｒ_a＜−０．０２， ｒ_a＜０
ただし、ｆは対物レンズ全系の焦点距離、ｒ_aは前群の負レンズの物体側の面の曲率半径である。
【０１１３】
（２） 特許請求の範囲の請求項１に記載する光学系で、下記条件（２）を満足することを特徴とする撮影光学系。
（２） ０．１３＜ｄ_F／ｆ＜１
ただし、ｄ_Fは前群の最も物体側に位置するレンズの像側の面から明るさ絞りまでの距離で、空気以外の媒質がある場合も含めて実寸法である。
【０１１４】
（３） 特許請求の範囲の請求項２に記載する光学系で、前記後群の正レンズが物体側の面が平面である平凸レンズであることを特徴とする撮影光学系。
【０１１５】
（４） 特許請求の範囲の請求項２に記載する光学系で、前記後群の接合レンズは像側のレンズが負レンズであることを特徴とする撮影光学系。
【０１１６】
（５） 特許請求の範囲の請求項２に記載する光学系で、下記条件（２）を満足することを特徴とする撮影光学系。
（２） ０．１３＜ｄ_F／ｆ＜１
ただし、ｄ_Fは前群の最も物体側に位置するレンズの像側の面から明るさ絞りまでの距離で、空気以外の媒質がある場合も含めて実寸法である。
【０１１７】
（６） 物体側から順に、外部に露出するカバーガラスと、対物レンズとを少なくとも含み、前記カバーガラスがオートクレーブ耐性を有する材質からなり、前記対物レンズが物体側から順に、負の屈折力を持つ前群と、明るさ絞りと、正の屈折力を持つ後群とにて構成され、前群の最も物体側の面が物体側に凹面を向け、下記条件（１−１）を満足する撮影光学系。
（１−１） −１．５＜ｆ／ｒ_a＜−０．０２， ｒ_a＜０
ただし、ｆは対物レンズ全系の焦点距離、ｒ_aは前群の負レンズの物体側の面の曲率半径である。
【０１１８】
（７） 前記の（６）の項に記載する光学系で、下記条件（２）を満足することを特徴とする撮像光学系。
（２） ０．１３＜ｄ_F／ｆ＜１
ただし、ｄ_Fは前群の最も物体側に位置するレンズの像側の面から明るさ絞りまでの距離で、空気以外の媒質がある場合も含めて実寸法である。
【０１１９】
（８） 物体側から順に、負の屈折力を持つ前群と、明るさ絞りと、正の屈折力を持つ後群とからなり、前群が物体側に凹面を向けた負レンズよりなり、後群が物体側から順に、物体側が平面の平凸レンズと、像側が負レンズの接合レンズとからなり、下記条件（３）、（４−１）、（５）を満足する対物レンズ。
（３） −０．９５＜（ｒ_b＋ｒ_a）／（ｒ_b−ｒ_a）＜１．０５
（４−１） ２＜ｆＢ／ＩＨ＜２．４
（５） ν_d（Ｃ）＜１９
ただし、ｒ_aは前群の負レンズの物体側の面の曲率半径、ｒ_bは前群の負レンズの像側の面の曲率半径、ｆＢは対物レンズの最も像側の面から後ろ側焦点位置までの距離、ＩＨは最大像高、ν_d（Ｃ）は接合レンズの負レンズのアッベ数である。
【０１２０】
（９） 物体側から順に、負の屈折力を持つ前群と、明るさ絞りと、正の屈折力を持つ後群とからなり、前群が物体側に凹面を向けた負レンズよりなり、後群が物体側から順に、物体側が平面の平凸レンズと、像側が負レンズの接合レンズとからなり、下記条件（２）、（３）、（４−１）、（５）を満足する対物レンズ。
（２） ０．１３＜ｄ_F／ｆ＜１
（３） −０．９５＜（ｒ_b＋ｒ_a）／（ｒ_b−ｒ_a）＜１．０５
（４−１） ２＜ｆＢ／ＩＨ＜２．４
（５） ν_d（Ｃ）＜１９
ただし、ｒ_aは前群の負レンズの物体側の面の曲率半径、ｒ_bは前群の負レンズの像側の面の曲率半径、ｆＢは対物レンズの最も像側の面から後ろ側焦点位置までの距離、ＩＨは最大像高、ν_d（Ｃ）は接合レンズの負レンズのアッベ数である。
【０１２１】
（１０） 物体側から順に、少なくとも外部に露出するカバーガラスと、対物レンズとからなり、前記カバーガラスがオートクレーブ耐性を有する材質からなり、前記対物レンズが、物体側より順に、負の屈折力を持つ前群と、明るさ絞りと、正の屈折力を持つ後群とよりなり、前群が物体側に凹面を向けた負レンズからなり、後群が物体側から順に正レンズと接合レンズとよりなり、前記対物レンズが、下記条件（１−１）、（２）、（３）、（４−１）、（５）を満足することを特徴とする請求項１の撮影光学系。
（１−１） −１．５＜ｆ／ｒ_a＜−０．０２， ｒ_a＜０
（２） ０．１３＜ｄ_F／ｆ＜１
（３） −０．９５＜（ｒ_b＋ｒ_a）／（ｒ_b−ｒ_a）＜１．０５
（４−１） ２＜ｆＢ／ＩＨ＜２．４
（５） ν_d（Ｃ）＜１９
ただし、ｆは対物レンズ全系の焦点距離、ｒ_aは前群の負レンズの物体側の面の曲率半径、ｒ_bは前群の負レンズの像側の面の曲率半径、ｄ_Fは前群の最も物体側に位置するレンズの像側の面から明るさ絞りまでの距離でその間にフィルターが配置されている場合も含め実寸法、ｆＢは対物レンズの最も像側の面から後ろ側焦点位置までの距離、ＩＨは最大像高、ν_d（Ｃ）は接合レンズの負レンズのアッベ数である。
【０１２２】
（１１） 前記の（９）の項に記載する光学系で、接合レンズが物体側より順に、正レンズと負レンズよりなることを特徴とする撮影光学系。
【０１２３】
（１２） 前記の（１０）の項に記載する光学系で、接合レンズが物体側より順に、正レンズと負レンズよりなることを特徴とする撮影光学系。
【０１２４】
（１３） 前記の（６）の項に記載する光学系で、撮影光学系が砒素、鉛等の有害物質を含まない材質からなることを特徴とする撮影光学系。
【０１２５】
（１４） 前記の（８）の項に記載する光学系で、撮影光学系が砒素、鉛等の有害物質を含まない材質からなることを特徴とする撮影光学系。
【０１２６】
（１５） 特許請求の範囲の請求項（１）、（２）又は（３）あるいは前記の（１）乃至（１４）の項のいずれかの項に記載する撮影光学系を用いた内視鏡。
【０１２７】
【発明の効果】
本発明によれば、オートクレーブ滅菌への耐性を有しており、ゴースト光が目立たない撮影光学系およびそれを用いた内視鏡を実現し得る。また、小型で結像性能がよく、加工性のよい撮影光学系およびそれを用いた内視鏡を実現し得る。
【図面の簡単な説明】
【図１】 本発明の撮影光学系の実施例１の断面図
【図２】 本発明の撮影光学系の実施例２の断面図
【図３】 本発明の撮影光学系の実施例３の断面図
【図４】 本発明の撮影光学系の実施例４の断面図
【図５】 本発明の撮影光学系の実施例５の断面図
【図６】 本発明の撮影光学系の実施例１の収差曲線図
【図７】 本発明の撮影光学系の実施例２の収差曲線図
【図８】 本発明の撮影光学系の実施例３の収差曲線図
【図９】 本発明の撮影光学系の実施例４の収差曲線図
【図１０】 本発明の撮影光学系の実施例５の収差曲線図
【図１１】 カバーガラスと対物レンズよりなる撮影光学系の通常光とゴースト光の結像等を示す図
【図１２】 本発明の実施例１の正規光の光路図
【図１３】 本発明の実施例１のゴースト光の光路図
【図１４】 カバーガラスを用いない光学系の構成を示す図
【図１５】 Ｓ−ＢＳＬ７のカバーガラスを用いた光学系の構成を示す図
【図１６】 本発明の実施例５の正規光の光路図
【図１７】 本発明の実施例５のゴースト光の光路図
【図１８】 画面上の正規光とゴースト光の結像位置を示す図
【図１９】 従来の撮影光学系の正規光の光路図
【図２０】 図１９に示す従来の撮影光学系のゴースト光の光路図[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a photographing optical system and an endoscope using the photographing optical system.
[0002]
[Prior art]
Today, endoscopes are widely used in the medical field to insert a long and thin insertion portion into a body cavity to observe a deep portion in the body cavity, and to perform treatment, processing, etc. using a processing tool as necessary. ing.
[0003]
In order to prevent infectious diseases and the like, it is essential that these endoscopes be sterilized and sterilized after use.
[0004]
Recently, sterilization using high-temperature and high-pressure steam, called autoclave sterilization, which can be used immediately after sterilization without complicated work and has low running cost, is becoming mainstream in sterilization of endoscope devices.
[0005]
In addition, in order to improve operability, medical endoscopes are desired to have a reduced diameter of the endoscope insertion portion or a shortened distal end hard portion of the endoscope insertion portion. Therefore, it is essential that the objective lens used in these medical endoscopes has a practically sufficient aberration correction and that the lens has a small outer diameter and a short overall length. It is.
[0006]
As a conventional example of a photographing optical system having resistance to autoclave sterilization as described above, there is one described in Japanese Patent Application No. 2001-173908. Further, as another conventional photographing optical system, for example, one described in Japanese Patent Laid-Open No. 2001-260347 is known.
[0007]
[Problems to be solved by the invention]
Imaging optical systems that are resistant to autoclave sterilization have the disadvantage of being prone to ghosting.
[0008]
The photographing optical system of the fifth example of Japanese Patent Application No. 2001-173908 uses a cover glass and has a configuration resistant to autoclave sterilization. The photographing optical system includes, in order from the object side, a first lens group including a cover glass made of a sapphire parallel plate, a plano-concave lens having a concave surface facing the image surface, and a plano-convex lens having a convex surface facing the image side. The second lens group includes a third lens group including a cemented lens of a biconvex positive lens and a negative lens.
[0009]
In this conventional optical system, ghost light caused by a cover glass is generated when a subject with high brightness is photographed.
[0010]
FIG. 19 shows an imaging optical path of normal light that is emitted from an object point and incident on the photographing optical system at an incident angle (angle formed by the optical axis and the light beam) of 10 ° in this conventional photographing optical system.
[0011]
FIG. 20 is a diagram showing an imaging optical path of ghost light that is emitted from an object point and enters the imaging optical system at an incident angle of 10 ° in the same conventional optical system. The ghost light is reflected by the object side surface of the objective lens and then reflected again by the object surface of the cover glass, and is imaged by the objective lens.
[0012]
The imaging position of the ghost light shown in FIG. 20 is close to the imaging position of the normal light shown in FIG. 19 and the imaging position on the screen and the imaging position on the optical axis.
[0013]
As described above, when the imaging position of the ghost light on the optical axis is close to the imaging position of the normal light on the optical axis, the ghost image is in focus and the ghost image is clearly observed.
[0014]
The conventional optical system shown in FIG. 19 uses sapphire as a cover glass. Since this sapphire has a refractive index of 1.7682 and a high refractive index, the reflectance of light is high and ghosts are easily noticeable.
[0015]
Further, in Examples other than the fifth example of Japanese Patent Application No. 2001-173908, the cover glass is not used, and the first lens of the objective lens is formed of sapphire. For this reason, this optical system does not generate ghost, but since the sapphire is extremely hard, it is difficult to process it into a lens, which increases the cost.
[0016]
Further, the endoscope photographing optical system is small in size, and it is generally difficult to process a lens even when sapphire is not used. For example, the first lens of Example 6 of Japanese Patent Application No. 2001-173908 has a small radius of curvature on the image side surface and is difficult to process.
[0017]
The present invention has been made in view of the above problems of the conventional example, and is an imaging optical system having resistance to autoclave sterilization, an imaging optical system in which ghost light is not noticeable, and an endoscope provided with this imaging optical system. A mirror is provided.
[0018]
The present invention also provides a photographing optical system that is compact and has good imaging performance and that has good processability, and an endoscope that includes this photographing optical system.
[0019]
[Means for Solving the Problems]
The first configuration of the photographing optical system of the present invention, in order from the object side, is composed of at least a cover glass exposed to the outside and an objective lens, the cover glass is made of a material having autoclave resistance, and the objective lens is In order from the object side, it consists of a front group with negative refractive power, an aperture stop, and a rear group with positive refractive power, with the most object side surface of the front group facing the concave side toward the object side It is a thing.
[0020]
The photographic optical system of the present invention having such a configuration can suppress the occurrence of a ghost despite the fact that a cover glass made of an autoclave-resistant material is disposed on the object side.
[0021]
Next, this point will be described based on the case where the object point is at infinity in an optical system similar to the thin-walled optical system shown in FIG.
[0022]
FIG. 11 shows an optical system composed of the cover glass CG and the objective lens OBL, and the imaging positions of normal light and ghost light by this optical system. Among these, the objective lens OBL is replaced with a thin lens and is indicated by an arrow in the figure. The focal length of the objective lens OBL is f.
[0023]
Usually, as shown in FIG. 11A, the light beam forms an image on P ′ on the rear focal position FB ′ by the objective lens OBL (normal light R).
[0024]
    Further, as shown in FIG. 11B, the ghost light GR is reflected on the most object side surface (S1) of the front group of the objective lens OBL, and the distance | on the object side of the objective lens OBL |r ₁An image is formed once at the position of | / 2.
[0025]
    Next, as shown in FIG. 11C, the light beam is reflected by the object side cover glass CG of the objective lens OBL, and the image side distance |r ₁An image is formed at a position Q of | / 2.
[0026]
Further, the image is re-imaged by the objective lens OBL, and finally formed at a position Q ′ at a distance b from the objective lens OBL. Here, as shown in FIG. 11D, the point Q ′ is shifted to the object side from the point P ′.
[0027]
The difference Δ in image position between the ghost light GR and the normal light R in the optical axis direction is as follows according to paraxial calculation.
[0028]
Δ = Q′−P ′
-1 / a + 1 / b = 1 / f
However, a is an object point distance and b is an image point distance. Substituting a = 1/2 × | r1 | into the above equation to obtain the value of b is as follows.
b = (f × | r1 |) / (| r1 | + 2 × f)
[0029]
Therefore, Δ is expressed as follows.
Δ = f × (2 × f / (| r1 | + 2 × f))
[0030]
The ratio Δ / f with respect to the focal length is as follows.
Δ / f = 2 × f / (| r1 | + 2 × f)
[0031]
As described above, the object point of the ghost light GR for the objective lens OBL is moved farther than the object point of the normal light R due to the reflection of the first surface S1 and the cover glass CG, thereby forming an image. The position has shifted to the object point side.
[0032]
As a result, the imaging position of the ghost light can be shifted from the image plane, and the ghost image is blurred and not clear.
[0033]
In addition, the imaging optical system of the present invention has an imaging position on the screen (imaging position in a direction perpendicular to the optical axis) outside the screen (a direction away from the optical axis) from the imaging position of normal light. Shift. As a result, ghost light is less likely to be generated.
[0034]
The objective lens of the photographing optical system according to the present invention is a retrofocus type including a front group having negative refractive power, an aperture stop, and a rear group having positive refractive power. Such a retrofocus type lens system has a large distance from the first surface to the principal point position of the entire objective lens system. Therefore, the image formation position by reflection of the concave surface of the first surface of the objective lens and the cover glass and the principal point position of the objective lens tend to be close. As a result, the imaging magnification of the objective lens becomes larger than the estimation with the thin lens. The direction in which the imaging magnification of the ghost image by the objective lens increases is the direction away from the optical axis. If a high-luminance subject is in the periphery of the screen, the ghost is off the screen and is not observed.
[0035]
For the above reasons, the photographic optical system having the above-described configuration according to the present invention can obtain a good observation image in which a ghost image is not conspicuous even for a subject having a high-luminance portion on the screen.
[0036]
In addition to the ghost light shown in FIG. 11C, the photographing optical system of the present invention described above is reflected by the surface S1 closest to the object side of the objective lens and further reflected by the image side surface of the cover glass. It is also possible to improve the ghost light that is imaged in step (b).
[0037]
In the photographic optical system according to the present invention, the surface closest to the object side of the objective lens (the surface closest to the object side in the front group) has a negative refractive power. It is possible to share the refractive power. Therefore, the radius of curvature of the other surfaces of the front group can be increased, and the lens can be easily processed.
[0038]
Furthermore, in the photographing optical system of the present invention, even if an optical member having high resistance to high-temperature and high-pressure steam such as synthetic quartz, permeable YAG, and spinel is used instead of sapphire as a material having autoclave resistance, the occurrence of ghost is suppressed. And a good observation image can be obtained.
[0039]
In the photographing optical system having the first configuration according to the present invention, it is preferable that the following condition (1) is satisfied.
(1) -2 <f / r_a<-0.02, r_a<0
Where f is the focal length of the entire objective lens system, r_aIs the radius of curvature of the object side surface of the negative lens in the front group.
[0040]
Condition (1) defines the refractive power of the most object side surface of the front group.
[0041]
In this condition (1), f / r_aWhen the value exceeds the upper limit of −0.02, the defocus amount of the ghost image is too small, and the improvement effect of the ghost light is small. F / r_aExceeds the lower limit of −2 of condition (1), which is advantageous for improving ghost light._a| Becomes smaller and it becomes difficult to obtain a wide angle of view.
[0042]
F / r_aIs −0.02 which is the upper limit value defined in the condition (1), Δ / f≈0.04 in the thin optical system, and the image position of the ghost light of about 4% of the focal length can be shifted.
[0043]
Although the estimated value in the thin optical system is different from the value in the actual optical system that is a thick lens, the defocus direction of the ghost image is always Q ′ closer to the object side than P ′.
[0044]
In the optical system having the above configuration, it is more preferable if the following condition (1-1) is satisfied instead of the condition (1).
(1-1) -1.5 <f / r_a<-0.02, r_a<0
[0045]
If the lower limit is set to −1.5 as in this condition (1-1), the ghost light is further improved and it becomes easy to secure a wide angle of view with an angle of view (2ω) of 70 ° or more. It is preferable as an imaging lens for an endoscope.
[0046]
Next, the second configuration of the present invention is characterized in that the objective lens is as follows.
[0047]
The objective lens according to the present invention includes a negative lens whose front group has a concave surface facing the object side, and the rear group includes a positive lens and a cemented lens in order from the object side. The objective lens satisfies the following condition (3): , (4), (5) are preferably satisfied.
(3) -0.95 <(r_b+ R_a) / (R_b-R_a) <1.05
(4) 1.5 <fB / IH <2.7
(5) ν_d(C) <19
Where r_aIs the radius of curvature of the object side surface of the negative lens in the front group, r_bIs the radius of curvature of the image side surface of the negative lens in the front group, fB is the distance from the most image side surface of the objective lens to the back focal position, IH is the maximum image height, ν_d(C) is the Abbe number of the negative lens of the cemented lens.
[0048]
Condition (3) defines the radius of curvature of the object-side and image-side surfaces of the negative lens in the front group, and is provided to improve the workability of this negative lens and to ensure imaging performance.
[0049]
In order to satisfy this condition (3), the negative refractive power of the negative lens in the front group is shared by both surfaces, thereby increasing the radius of curvature and improving the workability of the lens. In addition, astigmatism can be satisfactorily corrected by making the negative lens into a shape that satisfies the condition (3).
[0050]
    In this condition (3), if the lower limit value of −0.95 is exceeded, the negative refractive power of the negative lens will not be shared by both surfaces, and the processability of the lens cannot be improved. In addition, astigmatism is undercorrected,MeridionalThe image plane tends to be negative. If the upper limit of 1.05 of the condition (3) is exceeded, the effect of sharing the negative refractive power of the negative lens on both sides is lost, and the processability of the lens is not improved. In addition, the radius of curvature of the object side surface of the negative lens is small, which is disadvantageous for widening the angle of view. In addition, astigmatism tends to be overcorrected, and the meridional image plane tends to be positive.
[0051]
In order to reduce the size of the endoscope, it is desirable to satisfy the condition (4). That is, by ensuring the back focus so as to satisfy the condition (4), it is possible to achieve both the downsizing of the endoscope and the space for arranging the filter.
[0052]
If the value of fB / IH exceeds the upper limit of 2.7 of the condition (4), the back focus becomes larger than necessary, which is disadvantageous for downsizing. When the value of fB / IH exceeds the lower limit of 1.5 of the condition (4), it is advantageous for downsizing, but the back focus becomes small. For this reason, the space for arranging the filter and the like is not preferred.
[0053]
It is more preferable that the following condition (4-1) is satisfied instead of condition (4).
(4-1) 2 <fB / IH <2.4
[0054]
If the upper limit value and the lower limit value of the value of fB / IH are set to 2.4 and 2, respectively, as in the condition (4-1), it is more preferable for achieving both downsizing and securing of an arrangement space such as a filter. .
[0055]
Condition (5) defines the Abbe number of the negative lens of the rear group cemented lens, and relates to correction of chromatic aberration and workability of the cemented lens.
[0056]
When the front group having a negative refractive power is composed of only a negative lens as in the objective lens of the endoscope of the present invention, the chromatic aberration of magnification tends to be insufficiently corrected. Therefore, it is important to arrange an effective glass material at an effective position.
[0057]
In the objective lens of the present invention, the effective arrangement position of the negative lens is on the image side of the aperture stop and away from the stop.
[0058]
Therefore, the present invention is a cemented lens in which a negative lens made of a glass material that satisfies the condition (5), which is an effective glass material for correcting chromatic aberration of magnification, is arranged on the image side of the aperture stop and on the most image side. By using this, it is possible to effectively correct chromatic aberration of magnification that tends to be insufficiently corrected.
[0059]
Further, it is preferable that the cemented lens is composed of a positive lens and a negative lens in order from the object side.
[0060]
Further, considering the workability of the cemented lens, it is not necessary to reduce the curvature radius of the cemented surface by using a glass material having a small Abbe number and increasing the Abbe number difference between the positive lens and the negative lens. That is, the Abbe number ν of the negative lens_dWhen (C) exceeds the upper limit value 19 of the condition (5), the chromatic aberration of magnification can be effectively corrected, and a cemented lens with good workability cannot be obtained.
[0061]
As described above, according to the second configuration of the present invention, it is possible to obtain an imaging optical system for an endoscope that is small in size and has good imaging performance, in particular, various aberrations including chromatic aberration of magnification are well corrected. .
[0062]
In the objective lens according to the second configuration of the present invention, when a cover glass is arranged on the object side to form a photographing optical system, the cover glass is provided by the distance between the objective lens and the cover glass and the thickness of the cover glass. The off-axis ray height on the object side surface of the lens becomes high, and the diameter of the cover glass tends to increase.
[0063]
In order to solve this problem, it is preferable to satisfy the following condition (2).
(2) 0.13 <d_F/ F <1
Where d_FIs the distance from the image-side surface of the lens located closest to the object side in the front group to the brightness stop, and includes the actual dimensions even when there is a medium other than air.
[0064]
This condition (2) is a condition for reducing the size of the photographing optical system using the cover glass.
[0065]
d_FWhen the value of / f exceeds 1 of the upper limit value of the condition (2), the off-axis ray height increases, and the diameter of the cover glass increases, which is not preferable. In the case of an optical system having a wide angle of view such as an endoscope, it is not particularly preferable. D_FIf / f exceeds the lower limit of 0.13 of the condition (2), it is preferable in that the diameter of the cover glass can be reduced. However, it is difficult to secure the back focus. Becomes difficult.
[0066]
Further, the third configuration of the present invention includes, in order from the object side, at least a cover glass exposed to the outside and an objective lens, the cover glass is made of a material having autoclave resistance, and the objective lens is sequentially from the object side. The front group consists of a negative lens with a negative refractive power, an aperture stop, and a rear group with positive refractive power. The front group consists of a negative lens with a concave surface facing the object side. The objective lens satisfies the following conditions (1) to (5). The objective lens satisfies the following conditions (1) to (5).
(1) -2 <f / r_a<-0.02, r_a<0
(2) 0.13 <d_F/ F <1
(3) -0.95 <(r_b+ R_a) / (R_b-R_a) <1.05
(4) 1.5 <fB / IH <2.7
(5) ν_d(C) <19
Where f is the focal length of the entire objective lens system, r_aIs the radius of curvature of the object side surface of the negative lens in the front group, r_bIs the radius of curvature of the image side surface of the negative lens in the front group, d_FIs the distance from the image side surface of the lens located closest to the object side in the front group to the brightness stop, including the case where a filter is disposed between them, and fB is the rear side from the most image side surface of the objective lens. Distance to side focal position, IH is maximum image height, ν_d(C) is the Abbe number of the negative lens of the cemented lens.
[0067]
The photographic optical system having the third configuration is characterized in that the rear group of the objective lens includes a positive lens and a cemented lens in which a positive lens and a negative lens are cemented.
[0068]
Further, the conditions (1) to (5) are satisfied.
[0069]
These conditions are as described above, and the condition (1) is the first surface r of the front group._ThreeIn order to increase the ghost light removal effect.
[0070]
Condition (2) is for realizing downsizing of the photographing optical system using the cover glass.
[0071]
Condition (3) defines the shape of the negative lens in the front group, and is provided for improving the workability of the lens.
[0072]
Further, the condition (4) is for ensuring the back focus so as to achieve both the space for arranging the filters and the miniaturization of the optical system.
[0073]
Furthermore, condition (5) is for correcting chromatic aberration of magnification.
[0074]
Moreover, the setting of the upper limit value and the lower limit value in each condition is based on the same reason as already described.
[0075]
DETAILED DESCRIPTION OF THE INVENTION
DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described based on illustrated examples.
[0076]
FIGS. 1 to 5 are diagrams showing the configurations of the first to fifth embodiments of the endoscope photographing optical system according to the present invention. These examples have the following data:
[0077]
Example 1

[0078]
Example 2

[0079]
Example 3

[0080]
Example 4

[0081]
Example 5

Where r₁, R₂, ... are the radius of curvature of each lens surface, d₁, D₂, ... are the thickness and air spacing of each lens, n₁, N₂,... Is the refractive index of each lens with respect to the d-line, ν₁, Ν₂,... Are Abbe numbers for the d-line of each lens. R₀Is the object plane, d₀Is the distance from the object surface to the cover glass.
[0082]
As shown in FIG. 1, the photographing optical system of Example 1 includes an objective lens including a cover glass C1, a front group GF having a negative refractive power, an aperture stop S, and a rear group GR having a positive refractive power. And filters F1 and F2, an image sensor cover glass C2 and an image sensor sealing glass C3, and the front group GF is a biconcave lens (r_Three~ R_Four) And the rear group GR is a positive lens (r₆~ R₇), A positive cemented lens (r₉~ R₁₁). Note that FS is a flare stop.
[0083]
In addition, Examples 2 to 5 are configured as shown in FIGS. 2 to 5, respectively, and have substantially the same lens configuration as Example 1.
[0084]
6 to 10 are aberration curve diagrams of Examples 1 to 5, respectively.
[0085]
The imaging light and ghost light in the optical systems of Examples 1 to 5 will be described.
[0086]
In the imaging optical system of Example 1, the imaging light path of normal light and the imaging light path of ghost light are shown in FIGS.
[0087]
In these figures, FIG. 12 shows a normal imaging optical path when a light beam emitted from an object point enters the photographing optical system (Example 1) of the present invention at an incident angle of 10 °.
[0088]
The paraxial imaging position in this case is the surface r₁₉The distance is 0.057 closer to the image side.
[0089]
FIG. 13 shows an imaging optical path of ghost light that is emitted from an object point and enters the imaging optical system of Embodiment 1 at an incident angle of 10 °.
[0090]
The paraxial imaging position of this ghost light is the 19th surface (r₁₉) At the object side of 0.965. That is, r in FIG.₁₄And r₁₅In the filter F.
[0091]
As described above, the imaging position Q ′ of the ghost light is shifted by 1.022 toward the object side from the imaging position P ′ of the normal light. Further, the image forming position on the screen (image forming position in a direction perpendicular to the optical axis) is shifted to the outside of the screen (in a direction away from the optical axis) from the layering position of normal light.
[0092]
As a result, the ghost is blurred and not clear. In addition, if a high-luminance subject is present at the periphery of the screen, it will not be observed outside the ghost screen.
[0093]
FIG. 18 shows the positional relationship between normal light and ghost light on the screen.
[0094]
In FIG. 18, R1 is an image of the normal light (imaging light) near the center of the screen, RG1 is a ghost due to the normal light R1, R2 is an image of the periphery of the screen of the normal light, and RG2 is a ghost due to the normal light R2.
[0095]
As shown in this figure, the ghost light RG1 is blurred because the image formation position is shifted from the screen in the optical axis direction. Further, the ghost light RG2 is out of focus and out of the screen.
[0096]
The above is the optical system in the apparatus of Example 1 that uses an autoclave.
[0097]
In the case of an apparatus that does not use an autoclave, the photographing optical system may have the configuration shown in the figure.
[0098]
FIG. 14 shows a photographing optical system that does not use a cover glass, and uses S-BSL7 (manufactured by OHARA INC.) As the glass material of the cover glass C2 of the image sensor.
[0099]
FIG. 15 uses S-BSL7 as the material for the cover glass C1 on the object side.
[0100]
Thus, the photographing optical system used in an apparatus that does not use an autoclave need not use sapphire as the cover glass material. Therefore, cost can be reduced by using B-BSL7 as the material of the cover glass.
[0101]
Further, in all the optical systems of the embodiments, the optical system can be made of a glass material that does not contain lead or arsenic.
[0102]
For example, in the first embodiment, the cover glass C1 is sapphire and the first lens (r_Three~ R_Four) And the second lens (r₆~ R₇) Is S-LAH58 (manufactured by OHARA INC.), The third lens (r₉~ R_Ten) Is S-NPH2 (manufactured by OHARA INC.), The filter F1 is white plate or S-BSL7 (manufactured by OHARA INC.), The filter F2 is CM5000 (manufactured by HOYA Corporation), the cover glass C2 is sapphire, and the imaging device sealing glass C3. Is S-BSL7 (manufactured by OHARA INC.).
[0103]
Therefore, it is a photographic optical system that is resistant to autoclave sterilization, has no noticeable ghosting, is compact, has good processability for individual lenses, and does not contain harmful lead or arsenic. is there. In addition, the photographing optical system of this embodiment can reduce the disposal cost even when it is collected after the end of its useful life and disassembled and discarded.
[0104]
Although the first embodiment has been described above, the photographing optical systems of the second to fourth embodiments also have a lens configuration similar to that of the first embodiment, and the glass materials to be used are the same glass materials.
[0105]
Therefore, it has the same characteristics as described in the first embodiment.
[0106]
Example 5 has a configuration as shown in FIG. The fifth embodiment is an optical system having a lens configuration similar to that of the other embodiments. f / r_ThreeIs close to the upper limit of the condition (1), but the ghost can also be improved in this embodiment. That is, the imaging position Q ′ of the ghost light is shifted to the object side from the imaging position P ′ of the normal light.
[0107]
FIG. 16 shows a normal imaging optical path that exits from an object point and enters the imaging optical system at an incident angle of 20 ° in the optical system of the fifth embodiment. The paraxial imaging position of this optical system is the 19th surface (r₁₉) To the image side from 0.082.
[0108]
FIG. 17 shows an imaging optical path of ghost light that is emitted from an object point and enters the imaging optical system at an incident angle of 20 ° in the fifth embodiment.
[0109]
In Example 5, the paraxial imaging position of ghost light is the 19th surface (r₁₉) Is a distance of 0.025 closer to the image side.
[0110]
In the fifth embodiment, the imaging position Q ′ of the ghost light is shifted by 0.057 toward the object side from the imaging position of the regular light.
[0111]
The present invention is as described in detail above, and in addition to the invention described in the claims, the invention described in the following items can also achieve the object.
[0112]
(1) An imaging optical system according to claim 1, wherein the imaging system satisfies the following condition (1).
(1) -2 <f / r_a<-0.02, r_a<0
Where f is the focal length of the entire objective lens system, r_aIs the radius of curvature of the object side surface of the negative lens in the front group.
[0113]
(2) An imaging optical system according to claim 1, wherein the following condition (2) is satisfied.
(2) 0.13 <d_F/ F <1
Where d_FIs the distance from the image-side surface of the lens located closest to the object side in the front group to the brightness stop, and includes the actual dimensions even when there is a medium other than air.
[0114]
(3) An optical system according to claim 2, wherein the positive lens in the rear group is a plano-convex lens having a plane on the object side.
[0115]
(4) An imaging optical system according to claim 2, wherein the cemented lens in the rear group is a negative lens on the image side.
[0116]
(5) An optical system according to claim 2 of claim, wherein the following condition (2) is satisfied.
(2) 0.13 <d_F/ F <1
Where d_FIs the distance from the image-side surface of the lens located closest to the object side in the front group to the brightness stop, and includes the actual dimensions even when there is a medium other than air.
[0117]
(6) In order from the object side, at least a cover glass exposed to the outside and an objective lens are included. The cover glass is made of a material having autoclave resistance, and the objective lens has negative refractive power in order from the object side. An imaging system that includes a front group, an aperture stop, and a rear group having a positive refractive power, and the most object side surface of the front group faces a concave surface toward the object side, and satisfies the following condition (1-1) Optical system.
(1-1) -1.5 <f / r_a<-0.02, r_a<0
Where f is the focal length of the entire objective lens system, r_aIs the radius of curvature of the object side surface of the negative lens in the front group.
[0118]
(7) An imaging optical system that satisfies the following condition (2) in the optical system described in the item (6).
(2) 0.13 <d_F/ F <1
Where d_FIs the distance from the image-side surface of the lens located closest to the object side in the front group to the brightness stop, and includes the actual dimensions even when there is a medium other than air.
[0119]
(8) In order from the object side, the front group includes a negative lens having a negative refractive power, an aperture stop, and a rear group having a positive refractive power. The front group includes a negative lens with a concave surface facing the object side. An objective lens in which the rear group is composed of a plano-convex lens having a flat surface on the object side and a cemented lens having a negative lens on the image side, and satisfies the following conditions (3), (4-1), and (5).
(3) -0.95 <(r_b+ R_a) / (R_b-R_a) <1.05
(4-1) 2 <fB / IH <2.4
(5) ν_d(C) <19
Where r_aIs the radius of curvature of the object side surface of the negative lens in the front group, r_bIs the radius of curvature of the image side surface of the negative lens in the front group, fB is the distance from the most image side surface of the objective lens to the back focal position, IH is the maximum image height, ν_d(C) is the Abbe number of the negative lens of the cemented lens.
[0120]
(9) In order from the object side, the front group includes a negative lens having a negative refractive power, an aperture stop, and a rear group having a positive refractive power. The front group includes a negative lens with a concave surface facing the object side. An objective that satisfies the following conditions (2), (3), (4-1), and (5) consisting of a plano-convex lens having a plane on the object side and a cemented lens having a negative lens on the image side in order from the object side. lens.
(2) 0.13 <d_F/ F <1
(3) -0.95 <(r_b+ R_a) / (R_b-R_a) <1.05
(4-1) 2 <fB / IH <2.4
(5) ν_d(C) <19
Where r_aIs the radius of curvature of the object side surface of the negative lens in the front group, r_bIs the radius of curvature of the image side surface of the negative lens in the front group, fB is the distance from the most image side surface of the objective lens to the back focal position, IH is the maximum image height, ν_d(C) is the Abbe number of the negative lens of the cemented lens.
[0121]
(10) In order from the object side, at least a cover glass exposed to the outside and an objective lens, the cover glass is made of a material having autoclave resistance, and the objective lens has negative refractive power in order from the object side. A front lens group, an aperture stop, and a rear lens group having a positive refractive power.The front lens group includes a negative lens having a concave surface facing the object side. The photographic optical system according to claim 1, wherein the objective lens satisfies the following conditions (1-1), (2), (3), (4-1), and (5).
(1-1) -1.5 <f / r_a<-0.02, r_a<0
(2) 0.13 <d_F/ F <1
(3) -0.95 <(r_b+ R_a) / (R_b-R_a) <1.05
(4-1) 2 <fB / IH <2.4
(5) ν_d(C) <19
Where f is the focal length of the entire objective lens system, r_aIs the radius of curvature of the object side surface of the negative lens in the front group, r_bIs the radius of curvature of the image side surface of the negative lens in the front group, d_FIs the distance from the image side surface of the lens located closest to the object side in the front group to the brightness stop, including the case where a filter is disposed between them, and fB is the rear side from the most image side surface of the objective lens. Distance to side focal position, IH is maximum image height, ν_d(C) is the Abbe number of the negative lens of the cemented lens.
[0122]
(11) An imaging optical system according to the item (9), wherein the cemented lens includes a positive lens and a negative lens in order from the object side.
[0123]
(12) In the optical system described in the above item (10), the cemented lens includes a positive lens and a negative lens in order from the object side.
[0124]
(13) An optical system according to item (6), wherein the imaging optical system is made of a material that does not contain harmful substances such as arsenic and lead.
[0125]
(14) The photographic optical system according to (8), wherein the photographic optical system is made of a material that does not contain harmful substances such as arsenic and lead.
[0126]
(15) An endoscope using the photographing optical system according to any one of claims (1), (2) or (3) or any one of the above items (1) to (14) .
[0127]
【The invention's effect】
ADVANTAGE OF THE INVENTION According to this invention, it has the tolerance to autoclave sterilization and can implement | achieve the imaging optical system and endoscope using the same with which ghost light is not conspicuous. In addition, it is possible to realize a photographing optical system that is small in size, has good imaging performance, and good workability, and an endoscope using the same.
[Brief description of the drawings]
FIG. 1 is a cross-sectional view of Embodiment 1 of a photographing optical system according to the present invention.
FIG. 2 is a sectional view of Embodiment 2 of the photographing optical system of the present invention.
FIG. 3 is a cross-sectional view of Embodiment 3 of the photographing optical system of the present invention.
FIG. 4 is a sectional view of Embodiment 4 of the photographing optical system of the present invention.
FIG. 5 is a sectional view of Embodiment 5 of the photographing optical system of the present invention.
FIG. 6 is an aberration curve diagram of Example 1 of the photographing optical system according to the present invention.
FIG. 7 is an aberration curve diagram of Example 2 of the photographing optical system of the present invention.
FIG. 8 is an aberration curve diagram of Example 3 of the photographing optical system of the present invention.
FIG. 9 is an aberration curve diagram of Example 4 of the photographing optical system of the present invention.
FIG. 10 is an aberration curve diagram of Example 5 of the photographing optical system according to the present invention.
FIG. 11 is a diagram showing image formation of normal light and ghost light of a photographing optical system composed of a cover glass and an objective lens.
FIG. 12 is an optical path diagram of normal light according to the first embodiment of the present invention.
FIG. 13 is an optical path diagram of ghost light according to the first embodiment of the present invention.
FIG. 14 is a diagram showing the configuration of an optical system that does not use a cover glass.
FIG. 15 is a diagram showing a configuration of an optical system using a cover glass of S-BSL7.
FIG. 16 is an optical path diagram of regular light according to the fifth embodiment of the present invention.
FIG. 17 is an optical path diagram of ghost light according to the fifth embodiment of the present invention.
FIG. 18 is a diagram showing the imaging positions of regular light and ghost light on the screen.
FIG. 19 is a path diagram of regular light in a conventional photographing optical system.
20 is an optical path diagram of ghost light in the conventional photographing optical system shown in FIG.

Claims (12)

The objective lens is composed of, in order from the object side, a front group having a negative refractive power, an aperture stop, and a rear group having a positive refractive power, and the front group has a concave surface facing the object side. The rear lens group includes a positive lens and a cemented lens in order from the object side, and the objective lens satisfies the following conditions (3), (4), and (5).
(3) −0.95 <(r _b + r _a ) / (r _b −r _a ) <1.05
(4) 1.5 <fB / IH <2.7
(5) ν _d (C) <19
However, r _a is the radius of curvature of the object side surface of the negative lens of the front group, r _b is the radius of curvature of the image side surface of the negative lens of the front group, fB is back focal from the surface located nearest to the image side of the objective lens The distance to the position, IH is the maximum image height, and ν _d (C) is the Abbe number of the negative lens of the cemented lens. In order from the object side, at least a cover glass exposed to the outside and an objective lens, the cover glass is made of a material having autoclave resistance, and the objective lens has a negative refractive power in order from the object side. And an aperture stop and a rear group having a positive refractive power, the front group is composed of a negative lens with a concave surface facing the object side, and the rear group is composed of a positive lens and a cemented lens in order from the object side. The objective lens satisfies the following conditions (1), (2), (3), (4), and (5).
(1) -2 <f / r a <-0.02, r a <0
(2) 0.13 <d _F / f <1
(3) −0.95 <(r _b + r _a ) / (r _b −r _a ) <1.05
(4) 1.5 <fB / IH <2.7
(5) ν _d (C) <19
However, f is the focal length of the objective lens system, r _a is the radius of curvature of the object side surface of the negative lens of the front group, r _b is the radius of curvature of the image side surface of the negative lens of the front group, d _F before The actual size including the case where a filter is disposed between the distance from the image side surface of the lens located closest to the object side to the brightness stop in the group, and fB is the back focal point from the most image side surface of the objective lens. The distance to the position, IH is the maximum image height, and ν _d (C) is the Abbe number of the negative lens of the cemented lens. An optical system including the objective lens according to claim 1 , comprising a cover glass exposed to the outside of the optical system and satisfying the following condition (2):
Z (2) 0.13 <d _F / f <1
However, d _F is the distance from the image side surface of the lens located closest to the object side in the front group to the aperture stop, and is an actual size including the case where there is a medium other than air.     3. The optical system according to claim 2, wherein the positive lens in the rear group is a plano-convex lens having a plane on the object side.     3. The optical system according to claim 2, wherein the cemented lens in the rear group has a negative lens on the image side. In order from the object side, it consists of a front group with negative refractive power, an aperture stop, and a rear group with positive refractive power.The front group consists of a negative lens with a concave surface facing the object side. An objective lens that includes, in order from the object side, a planoconvex lens having a flat surface on the object side and a cemented lens having a negative lens on the image side, and satisfies the following conditions (3), (4-1), and (5).
(3) −0.95 <(r _b + r _a ) / (r _b −r _a ) <1.05
(4-1) 2 <fB / IH <2.4
(5) ν _d (C) <19
However, r _a is the radius of curvature of the object side surface of the negative lens of the front group, r _b is the radius of curvature of the image side surface of the negative lens of the front group, fB is back focal from the surface located nearest to the image side of the objective lens The distance to the position, IH is the maximum image height, and ν _d (C) is the Abbe number of the negative lens of the cemented lens. In order from the object side, it consists of a cover glass exposed to the outside, a front group with negative refractive power, an aperture stop, and a rear group with positive refractive power, and the front group has a concave surface facing the object side. It consists of a negative lens, and the rear group consists of a plano-convex lens with a flat surface on the object side and a cemented lens with a negative lens on the image side, and the following conditions (2), (3), (4-1), (5) ) Optical system that satisfies
(2) 0.13 <d _F / f <1
(3) −0.95 <(r _b + r _a ) / (r _b −r _a ) <1.05
(4-1) 2 <fB / IH <2.4
(5) ν _d (C) <19
However, r _a is the radius of curvature of the object side surface of the negative lens of the front group, r _b is the radius of curvature of the image side surface of the negative lens of the front group, fB is back focal from the surface located nearest to the image side of the objective lens The distance to the position, IH is the maximum image height, and ν _d (C) is the Abbe number of the negative lens of the cemented lens. In order from the object side, at least a cover glass exposed to the outside and an objective lens, the cover glass is made of a material having autoclave resistance, and the objective lens has a negative refractive power in order from the object side. And an aperture stop and a rear group having a positive refractive power, the front group is composed of a negative lens with a concave surface facing the object side, and the rear group is composed of a positive lens and a cemented lens in order from the object side, An imaging optical system in which the objective lens satisfies the following conditions (1-1), (2), (3), (4-1), and (5).
(1-1) -1.5 <f / r a <-0.02, r a <0
(2) 0.13 <d _F / f <1
(3) −0.95 <(r _b + r _a ) / (r _b −r _a ) <1.05
(4-1) 2 <fB / IH <2.4
(5) ν _d (C) <19
However, f is the focal length of the objective lens system, r _a is the radius of curvature of the object side surface of the negative lens of the front group, r _b is the radius of curvature of the image side surface of the negative lens of the front group, d _F before The actual size including the case where a filter is disposed between the distance from the image side surface of the lens located closest to the object side to the brightness stop in the group, and fB is the back focal point from the most image side surface of the objective lens. The distance to the position, IH is the maximum image height, and ν _d (C) is the Abbe number of the negative lens of the cemented lens. The optical system according to claim 7 , wherein the cemented lens is composed of a positive lens and a negative lens in order from the object side. 9. An optical system according to claim 8 , wherein the cemented lens is composed of a positive lens and a negative lens in order from the object side. 7. An optical system according to claim 6 , wherein the photographing optical system is made of a material that does not contain harmful substances such as arsenic and lead. An endoscope using the photographing optical system according to any one of claims 1 to 11 .

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