JP2004004928A - Image pickup lens - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To realize an image pickup lens consisting of two lenses and having excellent image-formation performance. <P>SOLUTION: The image pickup lens is constituted of a homogeneous lens having negative refractive power and a radial type refractive index distribution lens having positive refractive power, and both surfaces of the radial type refractive index distribution lens are plane, and one part or all of the surface of the homogeneous lens on the radial type refractive index distribution lens side is plane. By satisfying a condition (1/V<SB>10</SB><1/ν<SB>h</SB>), excellent optical performance is obtained with a small number of lenses. <P>COPYRIGHT: (C)2004,JPO

Description

【００８７】

【００８８】

【００８９】

【００９０】

【００９１】

【００９２】

【００９３】

ただしｒ_１，ｒ_２，・・・はレンズ各面の曲率半径、ｄ_１，ｄ_２，・・・は各レンズの肉厚およびレンズ間隔、ｎ_１，ｎ_２，・・・は各レンズのｄ線の屈折率、ν_１，ν_２，・・・は各レンズのアッベ数である。
【００９４】
本発明の実施例１は図１に示すものである。つまり、物体側より順に負レンズの第１レンズと正レンズの第２レンズの２枚構成で第２レンズが両平面のラジアル型屈折率分布レンズで構成された撮像レンズである。負の第１レンズは物体側の面が凹面で像側の面が平面形状であり、この平面部でラジアル型屈折率分布レンズと密着されている。また、絞りＳを均質レンズとラジアル型屈折率分布レンズの間に設けたことでレンズ系を通過する軸外光線の光線高を低くし、レンズの径を小さくするのに役立っている。
【００９５】
本実施例の撮像レンズは像面位置にＣＣＤ等の固体撮像素子を配置することで、例えばビデオカメラやテレビ電話あるいはインターホンに用いられる撮像系として利用することが可能である。また、特定の波長成分をカットするためのフィルターＦがラジアル型屈折率分布レンズの像側の面に接着あるいは密着されている。また、ラジアル型屈折率分布レンズあるいは均質レンズに特定の波長成分をカットするためのフィルター機能を持たせれば、図１中のフィルターＦは用いずに済ませることもできる。また、フィルターはラジアル型屈折率分布レンズと離して配置することも可能である。
【００９６】
また、実施例１では第１レンズを物体側に繰り出すことで至近距離物体へのフォーカシングを行う。そこで、均質レンズとラジアル型屈折率分布レンズとは接着せずに密着するような構成とした。
【００９７】
この実施例の近距離物体（物点位置１００ｍｍ）へのフォーカシング時の第１レンズと第２レンズとの間隔は３．８１０ｍｍで、絞りは第２レンズに密着されていて移動しない。
【００９８】
また、実施例１では均質レンズとラジアル型屈折率分布レンズの外径が等しくなっているので、両レンズの光軸を比較的高い精度で容易に一致させることが可能である。
【００９９】
また、本発明のレンズ系でフォーカシングを行わない場合は鏡枠等の簡素化のため均質レンズとラジアル型不均質レンズとを接着することもできる。
【０１００】
また、実施例１では均質レンズが負の屈折力を持つので、軸上色収差を良好に補正するためラジアル型屈折率分布レンズの１／Ｖ_１０は正の小さい値を持たせるようにした。
【０１０１】
また、本発明の撮像レンズを安価に作製することを考えると、この実施例１のように均質レンズは凹面と平面形状であることが望ましい。片側の面が平面形状であれば研磨が極めて容易であり、安価に加工することが可能である。
【０１０２】
また、本発明の撮像レンズは直接球面加工されたラジアル型屈折率分布レンズにては持ち得ない収差補正のメリットを有する。図２１は縦軸を屈折率ｎ_ｄ、横軸をアッベ数ν_ｄとしたガラスマップであり、太線で囲われた領域は既存ガラスのおおよその存在範囲、矢印はラジアル型屈折率分布レンズの屈折率とアッベ数の変化、黒丸は光軸上の屈折率とアッベ数を表している。例えば、イオン交換法やゾルゲル法により作製されるラジアル型屈折率分布レンズ素材の光軸から周辺へ分布している屈折率とアッベ数の値は、素材の作製上、図２１中の矢印Ａで示されるようにほぼ既存ガラスの範囲内となる。そのため、例えば、図２１中の矢印Ｂで示すような、光軸上の屈折率とアッベ数が既存ガラスの範囲内であり、周辺に行くにしたがい屈折率とアッベ数が既存ガラスの範囲から外れるようなラジアル型屈折率分布レンズ素材の作製は極めて困難である。しかし、諸収差の補正上このような素材が望まれる場合もある。例えば、矢印Ｂで表される素材は分散が小さいので色収差の補正に有利な素材である。ところが、本発明の撮像レンズを用いれば、ラジアル型屈折率分布レンズが既存ガラスの範囲内の素材であったとしても、前述の既存ガラスの範囲から外れるような屈折率分布を持つ素材とほぼ同様の効果を得ることが可能である。具体的に、実施例１を用いて説明する。矢印Ａは実施例１のラジアル型屈折率分布レンズを表しＮ_００＝１．７５，Ｖ_００＝４５，Ｖ_１０≒２００の値を持ち軸上色収差ＰＡＣ_Ａは下記式で表される。
【０１０３】
ＰＡＣ_Ａ＝Ｋ（φ_ｓ／４５＋φ_ｍ／２００）
また、矢印Ｂの軸上色収差ＰＡＣ_Ｂは下式で表される。
【０１０４】
ＰＡＣ_Ｂ＝Ｋ（φ_ｓ／７０．２１＋φ_ｍ／２００）
ところが、本実施例１ではｎ_ｈ＝１．４８７４９，ν_ｈ＝７０．２１の均質レンズとラジアル型屈折率分布レンズで構成され、この組み合わせレンズの軸上色収差ＰＡＣ_Ｃは下式となる。
【０１０５】
ＰＡＣ_Ｃ＝Ｋ（φ_ｈ／７０．２１＋φ_ｍ／２００）
そこで、φ_ｈ≒φ_ｓとなるように均質レンズの屈折力を選べば、実際には作製が困難である矢印Ｂで表される素材とほぼ同様の色収差補正効果を持つ撮像レンズを達成することができる。このように、本発明の撮像レンズでは、所望の均質レンズを選択することで収差補正の自由度が大きくなるというメリットがある。
【０１０６】
実施例１の撮像レンズの収差状況は、図９，図１０に示す通りで、図９は無限遠物点、図１０は近距離物点に対するものである。これら収差図の通り、レンズ枚数が２枚であるにも拘らず諸収差が良好に補正されていることがわかる。
【０１０７】
本発明の実施例２は図２に示すものである。つまり、物体側より順に負レンズの第１レンズと正レンズの第２レンズの２枚構成で第２レンズが両平面のラジアル型屈折率分布レンズで構成された撮像レンズである。また、第１レンズは凹面を像側に向けたメニスカス形状であり、像面側の面の有効径よりも外周部に平面部１を有し、この平面部１でラジアル型屈折率分布レンズと接着あるいは密着されている。また、特定の波長成分をカットするためのフィルターＦがラジアル型屈折率分布レンズの像側に用いられている。
【０１０８】
実施例２は実施例１で用いた撮像レンズの均質レンズをメニスカス形状の均質レンズと交換したものであり、ラジアル型屈折率分布レンズは共通であるにもかかわらず、焦点距離の異なる撮像レンズを実現した例である。つまり、本発明の撮像レンズは図２２に示すようにラジアル型屈折率分布レンズＬ_ｇを共通としｒ，ｄ，ｎの異なる様々な形状の均質レンズ、例えばＬ_ｈ１，Ｌ_ｈ２，Ｌ_ｈ３を組み合わせることで、ラジアル型屈折率分布レンズが１種類であるにも拘らず焦点距離や諸収差の発生量が異なる撮像レンズＬ_ｃ１，Ｌ_ｃ２，Ｌ_ｃ３を達成することができる。このようにラジアル型屈折率分布レンズを共通部品として使えるため、白色光源下で用いる様々なレンズ系に対応できるにもかかわらず、安価なレンズ系を実現することが可能である。
【０１０９】
また、絞りｓを均質レンズの物体側に設けたことにより、特に軸外収差を良好に補正することが可能になった。
【０１１０】
また、本実施例は比較的焦点距離が長く、特に軸上色収差の補正が難しいことから、均質レンズにアッベ数が１／ν_ｈ＞０．０３を満足するような高分散ガラスを用いるようにした。また、絞りが最も物体側に設けられているので、均質レンズに高分散ガラスを用いたことは倍率の色収差を良好に補正する上でも望ましい。
【０１１１】
また、均質レンズの像面側の面に非球面を用いたことで、諸収差を良好に補正している。なお、実施例中で用いた非球面形状は以下の式で表されるものである。

【０１１２】
ただし、上記式はｘ軸を光軸方向にとり、ｙ軸を光軸と直角方向にとったもので、ｒは光軸上の曲率半径、Ｐは円錐定数、Ａ_２ｉは非球面係数である。
【０１１３】
実施例２も第１レンズを物体側へ移動させてフォーカシングを行なう。この実施例２の近距離物体（物点距離１００ｍｍ）へのフォーカシング時の第１レンズと第２レンズの間隔は１．５１５１ｍｍで、絞りは第１レンズとともに移動する。
【０１１４】
実施例２の収差状況は、図１１，図１２に示す通りで、図１１は無限遠物点、図１２は近距離物点に対するものである。この実施例２の撮像レンズは、レンズ枚数が２枚であるにも拘らず諸収差が良好に補正されていることがわかる。
【０１１５】
本発明の実施例３は図３に示すものである。つまり、物体側より順に負レンズの第１レンズと正レンズの第２レンズの２枚構成で第２レンズが両平面のラジアル型屈折率分布レンズで構成された撮像レンズである。また、第１レンズは平凹レンズであり、像側の面の有効径よりも外周部に平面部１を有し、この平面部１でラジアル型屈折率分布レンズと接着あるいは密着されている。また、絞りＳはラジアル型屈折率分布レンズの物体側の面から像側に向かって１ｍｍの位置に設けられている。また、ラジアル型屈折率分布レンズの像側に特定の波長成分をカットするためのフィルターＦが用いられている。
【０１１６】
実施例３は均質レンズの負の屈折力を大きくしたことによって、レンズ系の広角化を達成した例であり、例えば内視鏡の対物レンズ等に用いることが可能である。実施例３の如く本発明の撮像レンズを広画角なレンズに適用する場合は、下記の条件（１３）を満足することが望ましい。
【０１１７】
（１３）　　−１．５＜ｆ／ｆ_ｈ＜−０．４
もし、条件（１３）の上限値の−０．４を越えると広画角化を達成することが困難となる。また、下限値の−１．５を越えるとペッツバール和が補正過剰になり像面が物体から遠ざかる方向に倒れてくるため好ましくない。
【０１１８】
また、本実施例は広画角なレンズ系であるため特にペッツバール和の補正が困難であるがラジアル型屈折率分布レンズがＮ_００ｄ＞１．６を満足することでこれを良好に補正している。ただし、Ｎ_００ｄ＞１．６５を満足すればペッツバール和をさらに良好に補正できるのでより望ましい。
【０１１９】
また絞りをラジアル型屈折率分布レンズ内部に設けたことでレンズの径を小さくすることに加えて、特に軸外収差を良好に補正することが可能になった。
【０１２０】
また、本発明の撮像レンズの均質レンズとラジアル型屈折率分布レンズとを接着する場合、図３中に１で示す平面部に接着材を塗布して組立てもよいが、図３中の符号Ｂにて示す部分を図３（Ｂ）に４にて示すようにレンズの外周部に接着剤を塗布して接着することも可能である。
【０１２１】
実施例３の収差状況は、図１３に示す通りである。この実施例３の撮像レンズは、レンズ枚数が２枚であるにも拘らず諸収差が良好に補正されていることがわかる。
【０１２２】
本発明の実施例４は図４に示すものである。つまり、物体側より順に負レンズの第１レンズと正レンズの第２レンズの２枚構成で第２レンズが両平面のラジアル型屈折率分布レンズで構成された撮像レンズである。また、第１レンズは凹平形状のレンズであり、像面側でラジアル型屈折率分布レンズと接着あるいは密着されている。また、固体撮像素子あるいは固体撮像素子ユニットＣがラジアル型屈折率分布レンズの像側部分に接着あるいは密着しており、小型化あるいは鏡枠構成の簡素化に効果がある。
【０１２３】
また、画角が比較的狭いレンズ系に本発明の撮像レンズを適用する場合は、負の屈折力の均質レンズが極端に大きな値を持つことは好ましくなく、下記の条件（１４）を満足することが望ましい。
【０１２４】
（１４）　　−０．８＜ｆ／ｆ_ｈ＜−０．１
もし、条件（１４）の下限値の−０．８を越えるとペッツバール和が補正過剰となり、像面が物体から遠ざかる方向に倒れてくる。また、上限値の−０．１を越えるとペッツバール和が補正不足となり像面が物体側に倒れてくるため好ましくない。
【０１２５】
また、透過率やフレアーが問題となる光学系に本発明の撮像レンズを適用する場合は、ラジアル型屈折率分布レンズのレンズ全長を４０ｍｍ程度以下にすることが望ましい。また、本実施例の如くラジアル型屈折率分布レンズが２５ｍｍ程度以下であればさらに良好な結像性能を持つレンズ系を達成できる。
【０１２６】
また、レンズ外周側面でのフレアーを防止するためには、レンズの外周側面を梨地状にしたり、レンズ外周側面に黒色などの比較的濃い色の塗料をぬることが効果的である。
【０１２７】
実施例４の収差状況は、図１４に示す通りである。この図に示すように実施例４の撮像レンズは、レンズ枚数が２枚であるにも拘らず諸収差が良好に補正されていることがわかる。
【０１２８】
実施例５は図５に示す構成のものである。つまり、物体側より順に負レンズの第１レンズと正レンズの第２レンズの２枚構成で第２レンズが両平面のラジアル型屈折率分布レンズで構成された撮像レンズである。また、第１レンズは両凹形状のレンズであり、像面側の平面部１でラジアル型屈折率分布レンズと接着あるいは密着されている。また、明るさ絞りＳがラジアル型屈折率分布レンズの物体側の面から像面側へ５ｍｍの所に設けられている。さらに、フレアー成分等をカットするための視野絞りＦＳがラジアル型屈折率分布レンズに設けられている。これらの、絞りはラジアル型屈折率分布レンズの外周部から切り込みを入れて、切り込んだ面に光束を遮断するような塗料を塗るか、あるいはラジアル型屈折率分布レンズを切断して切断面に絞りを印刷、蒸着あるいは薄板を挟み込んで再度接着あるいは密着して作製できる。
【０１２９】
実施例５の収差状況は、図１５に示す通りである。この図に示すように実施例５の撮像レンズは、レンズ枚数が２枚であるにも拘らず諸収差が良好に補正されていることがわかる。
【０１３０】
本発明の実施例６は図６に示すものである。つまり、物体側より順に負レンズの第１レンズと正レンズの第２レンズの２枚構成で第２レンズが両平面のラジアル型屈折率分布レンズで構成された撮像レンズである。また、第１レンズは凹平形状のレンズであり、像側の面でラジアル型屈折率分布レンズと接着あるいは密着されている。
【０１３１】
また、ラジアル型屈折率分布レンズの直径を均質レンズの直径と異なる様にし又鏡枠あるいは鏡枠の一部６を効率的に配置できたことでレンズ系の小型化を達成している。
【０１３２】
実施例６の収差状況は図１６に示す通りである。この図の通り実施例６の撮像レンズではレンズ枚数が２枚であるにも拘らず諸収差が良好に補正されていることがわかる。
【０１３３】
本発明の実施例７は図７に示すものである。つまり、物体側より順に両平面の形状の正の屈折力を持つラジアル型不均質レンズの第１レンズと負の屈折力の均質レンズの第２レンズからなる。第２レンズの均質レンズは凹平形状であり物体側の面の有効径よりも外周部に平面部１を有し、この平面部でラジアル型不均質レンズと接着あるいは密着されている。また、絞りＳがラジアル型不均質レンズ物体側の面よりも像側に２．７３６９ｍｍの位置に設けられている。
【０１３４】
実施例７は等倍結像の光学系となっており、像面位置に固体撮像素子を配置すればビデオマイクロスコープ等の撮像レンズ系として用いることが可能である。
【０１３５】
実施例７の収差状況は図１７に示す通りである。この図のように実施例７の撮像レンズは、レンズ枚数が２枚であるにも拘らず諸収差が良好に補正されていることがわかる。
【０１３６】
実施例８は図８に示す構成のものである。つまり、物体側より順に負レンズの第１レンズと正レンズの第２レンズの２枚構成で第２レンズが両平面のラジアル型屈折率分布レンズで構成された撮像レンズである。また、第１レンズは両凹形状のレンズであり、像面側の平面部１とラジアル型屈折率分布レンズの平面部８で間隔環７を挟んでいる。また、明るさ絞りＳがラジアル型屈折率分布レンズ物体側の面から３．８９７３ｍｍの所に設けられている。第１レンズと第２レンズの間隔を変化させてフォーカシングを行う場合は、図８（Ｂ）において第１レンズ、間隔環７、ラジアル型屈折率分布レンズを接着せずに配置することが望ましいが、第１レンズと間隔環７、あるいは間隔環７とラジアル型屈折率分布レンズを接着することも可能である。また、実施例８では間隔環７が両平面の形状の薄板となっているが、例えば、図２３の斜線部で示す様に間隔環７を光軸に対して斜めの面を持つような形状にすることも可能である。図２３の場合は、間隔環７の平面部とラジアル型屈折率分布レンズＬ_ｇの平面部６とを密着させ密着面をスライドさせることにより、両レンズの光軸位置が一致するように調節することができる。また、図２４に示すように均質凹レンズの比較的鋭利な部位８をラジアル型屈折率分布レンズに密着させ、間隔環を介さずに両レンズの光軸を一致させるための調整をすることも可能である。
【０１３７】
また、より高い結像性能を達成するには均質レンズが本実施例の如く、両凹形状であることが望ましい。両凹形状であれば両面に負の屈折力を分配できるので、このレンズで発生する諸収差を小さくすることが可能である。
【０１３８】
この実施例８において、近距離物体（物点距離１００ｍｍ）へのフォーカシング時の第１レンズと第２レンズの間隔は０．６１４８ｍｍで、絞りは第２レンズ（屈折率分布レンズ）中に固定されている。
【０１３９】
実施例８の収差状況は、図１８に示す通りである。この図のように実施例８の撮像レンズは、レンズ枚数が２枚であるにも拘らず諸収差が良好に補正されていることがわかる。
【０１４０】
次に、本発明の撮像レンズを製作する場合について従来例と比較しながら図２５，図２６を用いて詳細に説明する。
【０１４１】
図２５は従来例を表しており、ラジアル型屈折率分布レンズに直接球面加工を施す場合のモデルである。図２５（Ａ）は球面加工前の円柱形状のラジアル型屈折率分布レンズと屈折率分布を表した図であり、図中Ｌ_ｇはラジアル型屈折率分布レンズ、ｎ（ｒ）は半径ｒの点での屈折率を表し、媒質の光軸１０は外周部１３の中心軸と一致している。図２５（Ｂ）は片側の面１４を球面に加工した場合の理想的な例であり、面と媒質の光軸は一致している。しかし、実際の加工の場面では図２５（Ｂ）の如く面Ｒ_１を高い精度で容易に加工することは困難であり、図２５（Ｃ）に示す様に、媒質の光軸１０と面Ｒ_１の光軸１５は、媒質の光軸と垂直な方向１６にδ、傾き角εの偏心が生じてしまう場合がある。均質レンズの場合はこの様な偏心が生じても、研磨面の光軸を衝としてレンズ外周を切削する心取り加工ができるため偏心は問題とならないが、ラジアル型屈折率分布レンズの場合は媒質の光軸が外周部に対して偏心してしまうため心取り加工ができない。
【０１４２】
そこで、本発明のレンズ系では図２６に示すモデルの如く、両平面の形状のラジアル型屈折率分布レンズと平面部１９を有する均質球面レンズを接着あるいは密着させて用いるようにした。図２６（Ａ）は、両面を平面加工したラジアル型屈折率分布レンズＬ_ｇと片側に平面部１９を有する均質レンズＬ_ｈを表している。ラジアル型屈折率分布レンズＬ_ｇの媒質の光軸１０は外周部１３の中心軸と一致しており、また、均質レンズＬ_ｈは心取り加工が可能なため面Ｒ_１の光軸１６は外周部２１の中心軸と高い精度で容易に一致させることができる。図２６（Ｂ）はラジアル型屈折率分布レンズＬ_ｇと均質レンズＬ_ｈとをそれぞれの平面部２０および１９とで接着あるいは密着させた図を表している。このとき、例えば、ラジアル型屈折率分布レンズの外周部１３と均質レンズＬ_ｈの外周部２１が一致するようにすれば、ラジアル型屈折率分布レンズの媒質の光軸と均質レンズの光軸とを容易に一致させることができる。
【０１４３】
また、図２５に示す従来の方法では球面加工の際に偏心が生じた場合、ラジアル型屈折率分布レンズＬ_ｇは利用できなくなるため歩留まりが悪くコスト高になるが、本発明の図２６に示す方法では前述したように、球面加工の際の偏心は全く問題にならないので歩留まりを高くして極めて安価に加工することが可能である。また、ラジアル型屈折率分布レンズも両面が平面であるため、高精度かつ安価に加工することができる。
【０１４４】
また、図２７は極めてシンプルな構造の鏡枠に本発明の撮像レンズを組み込んだ例である。図２７中Ｌ_ｈは凹レンズ、Ｌ_ｇはラジアル型屈折率分布レンズ、２５は鏡枠でこの場合は円筒形状である。また、２６，２７は固体撮像素子とその処理回路部を表し、２８はレンズ保護のためのカバーガラスである。図２７の様な構成とすれば、鏡枠２５に均質レンズ、ラジアル型屈折率分布レンズを落し込むことが出来るので組立が極めて容易である。また、本実施例の様に撮像レンズが鏡枠で保持される場合は、必ずしも均質レンズとラジアル型屈折率分布レンズとが接着されている必要はない。また、あらかじめ均質レンズとラジアル型屈折率分布レンズの心を合わせて接着し、その後鏡枠に組み込めば高い精度で組み立てることができる。また、このとき、撮像素子をあらかじめラジアル型屈折率分布レンズに接着することも可能である。
【０１４５】
また、カバーガラス２８を特定の波長成分をカットするための光学フィルターで構成することも可能である。
【０１４６】
また、図２８は本発明の撮像レンズでフォーカシングを行うための鏡枠を構造を表したものである。図２８中Ｌ_ｈは凹レンズ、Ｌ_ｇはラジアル型屈折率分布レンズ、２５は鏡枠でこの場合はラジアル型屈折率分布レンズＬ_ｇを保持している。また、２６，２７は固体撮像素子とその処理回路部を表し、２９は鏡枠で均質レンズＬ_ｈを保持している。図２８に示すように均質レンズＬ_ｈとラジアル型屈折率分布レンズＬ_ｇとが異なる鏡枠部品２５，２９で保持されていることでフォーカシングを行うことを可能としている。図２８では鏡枠２５と鏡枠２９とは通常、銀塩カメラ等の鏡枠で用いられるようなカム溝を介して接続した構成となっているが、ネジ部を介して接続するあるいは接触面をなめらかな形状とし片方の鏡枠がスライドしてフォーカシングを行っても良い。
【０１４７】
下記の各項に示す撮像レンズも、本発明の目的を達成し得るものである。
【０１４８】
（１）　特許請求の範囲の請求項１，２又は３に記載されているレンズ系で、下記条件（３）を満足する撮像レンズ。
【０１４９】
（３）　　−１．５＜ｆ／ｆ_ｆ＜−０．０５
（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）の項に記載されているレンズ系で、下記条件（４）を満足する撮像レンズ。
【０１５０】
（４）　　０．５＜ｔ_Ｇ／ｆ＜９
（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）の項に記載されているレンズ系で、下記条件（５）を満足する撮像レンズ。
【０１５１】
（５）　　１．５＜ｔ_Ｇ／ｆ＜７
（４）　特許請求の範囲の請求項２又は３あるいは前記の（１），（２）又は（３）の項に記載されているレンズ系で、下記条件（６）を満足する撮像レンズ。
【０１５２】
（６）　　−０．０１＜１／Ｖ_１０＜０．００８
（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）の項に記載されているレンズ系で、下記条件（７）を満足する撮像レンズ。
【０１５３】
（７）　　−０．２＜Ｎ_２０×ｆ^４＜０．２
（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）又は（４）の項に記載されているレンズ系で、下記条件（８）を満足する撮像レンズ。
【０１５４】
（８）　　−０．０５＜Ｎ_２０×ｆ^４＜０．０５
（７）　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），（５）又は（６）の項に記載されているレンズ系で、下記条件（９）を満足する撮像レンズ。
【０１５５】
（９）　　１／ν_ｈ＜０．０３
（８）　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），（５）又は（６）に記載されているレンズ系で、下記条件（１０）を満足する撮像レンズ。
【０１５６】
（１０）　　１／ν_ｈ＜０．０２５
（９）　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），（５），（６），（７）又は（８）に記載されているレンズ系で、下記条件（１１）を満足する撮像レンズ。
【０１５７】
（１１）　　Ｎ_００ｄ＞１．５５
（１０）　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），（５），（６），（７），（８）又は（９）に記載されているレンズ系で、下記条件（１２）を満足する撮像レンズ。
【０１５８】
（１２）　　０．５＜ａ_ＰＡＣ＜１．７
（１１）　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），（５），（６），（７），（８），（９）又は（１０）に記載されているレンズ系で、下記条件（１３）を満足する撮像レンズ。
【０１５９】
（１３）　　−１．５＜ｆ／ｆ_ｈ＜−０．４
（１２）　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），（５），（６），（７），（８），（９）又は（１０）に記載されているレンズ系で、下記条件（１４）を満足する撮像レンズ。
【０１６０】
（１４）　　−０．８＜ｆ／ｆ_ｈ＜−０．１
【０１６１】
【発明の効果】
本発明の撮像レンズは、加工性に優れていて諸収差、特に軸上色収差とペッツバール和が良好に補正されている。
【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】本発明の実施例６の断面図
【図７】本発明の実施例７の断面図
【図８】本発明の実施例８の断面図
【図９】本発明の実施例１の無限遠物点に対する収差曲線図
【図１０】本発明の実施例１の近距離物点（物点距離１００ｍｍ）に対する収差曲線図
【図１１】本発明の実施例２の無限遠物点に対する収差曲線図
【図１２】本発明の実施例２の近距離物点（物点距離１００ｍｍ）に対する収差曲線図
【図１３】本発明の実施例３の近距離物点（物点距離１１ｍｍ）に対する収差曲線図
【図１４】本発明の実施例４の無限遠物点に対する収差曲線図
【図１５】本発明の実施例５の無限遠物点に対する収差曲線図
【図１６】本発明の実施例６の無限遠物点に対する収差曲線図
【図１７】本発明の実施例７の近距離物点（物点距離２．８５ｍｍ）に対する収差曲線図
【図１８】本発明の実施例８の無限遠物点に対する収差曲線図
【図１９】本発明の実施例８の近距離物点（物点距離１００ｍｍ）に対する収差曲線図
【図２０】媒質の屈折力比ａと軸上色収差およびペッツバール和との関係を示すグラフ
【図２１】硝材の屈折率−アッベ数の分布を示す図
【図２２】ラジアル型屈折率分布レンズと均質レンズとの組合わせの例を示す図
【図２３】ラジアル型屈折率分布レンズの平面と均質レンズの凹面側との密着手段を示す図
【図２４】ラジアル型屈折率分布レンズの平面と均質レンズの凹面側との他の密着手段を示す図
【図２５】従来の製作例であるラジアル型屈折率分布レンズに直接球面加工を行なう例を示す図
【図２６】本発明の撮像レンズの組合わせ例を示す図
【図２７】本発明の撮像レンズの鏡枠への組込み例を示す図
【図２８】本発明の撮像レンズの鏡枠への組込みの他の例を示す図[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an imaging lens such as a video camera for performing electronic imaging.
[0002]
[Prior art]
2. Description of the Related Art In recent years, with the spread of video cameras, videophones, camera-equipped doorphones, and the like using a solid-state imaging device such as a CCD, further miniaturization and cost reduction are required for lens systems used in these cameras. In general, about 3 to 6 lenses are required to obtain good optical performance with a fixed-focus lens system used for these, but it is desired to further reduce the number of lenses.
[0003]
As a means for reducing the number of lenses while maintaining desired performance, it has been conventionally known to use a radial type gradient index lens element in an optical system. For example, as a conventional example of a lens system composed of a single radial type gradient index lens, a lens system described in Japanese Patent Application Laid-Open No. H6-175016 is known. However, in this conventional example, in order to improve the imaging performance, the F-number is extremely dark at 9.8 to 13.5 to balance various aberrations. As a conventional example of a lens system composed of two lenses, one disclosed in Japanese Patent Application Laid-Open No. Sho 60-218614 is known. In this conventional example, various aberrations are satisfactorily corrected, but the cost increases because two radial type gradient index lenses are used. In addition, since the radial refractive index distribution lens has a spherical shape, it is difficult to easily match the optical axis of the surface with the optical axis of the medium with high precision in actual processing. It is not preferable in terms of realizing or realizing a lens system whose manufacturing cost is low.
[0004]
In order to solve the above-mentioned drawback, it is conceivable to form the radial type gradient index lens into a biplanar shape, but it is not preferable because the Petzval sum is insufficiently corrected as compared with the radial type gradient index lens having a spherical shape.
[0005]
Japanese Patent Application Laid-Open Nos. 58-59420 and 4-1992 disclose a conventional example in which a radial refractive index distribution lens having two flat surfaces and a homogeneous lens having one flat surface and the other surface being spherically processed are used. A lens system described in, for example, Japanese Patent Publication No. 114112 is known. However, these prior arts relate to a pickup lens used under a monochromatic light source, and there is no description in the publication about correction of chromatic aberration. Similarly, Japanese Patent Application Laid-Open Nos. 1-28514 and 2-284107 do not disclose any correction of chromatic aberration.
[0006]
Further, as a conventional lens system in consideration of chromatic aberration, one described in Japanese Patent Application Laid-Open No. 58-184113 is known. In this conventional example, a radial type gradient index lens is used for a rigid mirror relay lens. This is an example. Since the refractive index difference between the optical axis of the radial type gradient index lens and the peripheral part is small and the lens length is very long, a special lens such as a rigid mirror relay lens that transmits an image multiple times with one lens is used. It can be used only for lens systems for various applications.
[0007]
As a lens system considering chromatic aberration, there is a conventional example described in JP-A-50-29238. In this conventional example, the refractive index difference from the optical axis to the periphery of a radial type gradient index lens is large. Therefore, it is difficult to actually manufacture a radial type gradient index lens material.
[0008]
[Problems to be solved by the invention]
SUMMARY OF THE INVENTION An object of the present invention is to provide a lens system having good imaging performance with two lenses.
[0009]
[Means for Solving the Problems]
The imaging lens according to the present invention includes a homogeneous lens having a negative refractive power and a radial refractive index distribution represented by the following equation (a) in which the refractive index is distributed in a radial direction from an optical axis having a positive refractive power. The lens system comprises a lens and satisfies the following condition (1).
[0010]
N (r) = N₀₀+ N₁₀r²+ N₂₀r⁴... (a)
(1) -0.05 <1 / V₁₀<0.01
Where r is the distance from the optical axis in the radial direction, N (r) is the refractive index distribution at the point of the distance r, N_i0Is the 2i order refractive index distribution coefficient, V_i0Represents the 2i-th order dispersion of the radial type gradient index lens and is given by the following equations (b) and (c).
[0011]
V₀₀= (N_00d-1) / (N_00F-N_00C) (I = 0) (b)
V_i0= N_i0d/ (N_i0F-N_i0C) {(I = 1, 2, 3,...)} (C)
Where N_00d, N_00F, N_00CIs the refractive index on the optical axis of the d, F, and C lines, respectively, N_i0d, N_i0F, N_i0CAre the 2i-order refractive index distribution coefficients of the d, F, and C lines, respectively.
[0012]
In order to solve the above-mentioned problems, the present invention has considered the following photographic lens having two lenses. That is, it is an imaging lens composed of one homogeneous lens and one radial type gradient index lens element. If the number of lenses is two, the cost required for polishing and assembly can be reduced, and the lens frame configuration can be simplified, so that an extremely inexpensive imaging lens can be achieved. In addition, by using a radial type gradient index lens element having a higher degree of freedom for aberration correction than a homogeneous lens, an imaging lens having high imaging performance can be achieved.
[0013]
Correction of chromatic aberration is particularly important for achieving an imaging lens with high imaging performance, and the radial type gradient index lens has excellent characteristics for correcting chromatic aberration.
[0014]
The axial chromatic aberration PAC of the radial type gradient index lens is approximated by the following equation (d).
[0015]
PAC = K (φ_S/ V₀₀+ Φ_m/ V₁₀) (D)
Where K is a constant dependent on the ray height of the on-axis ray and the final paraxial ray angle, φ_SAnd φ_mAre the refractive power of the thin wall of the surface of the radial type gradient index lens and the refractive power of the medium.
[0016]
As is clear from equation (d), V of the radial type gradient index lens element₁₀Is set to an appropriate value, the amount of longitudinal chromatic aberration can be controlled.
[0017]
The imaging lens of the present invention satisfies the above condition (1) in order to favorably correct axial chromatic aberration.
[0018]
If this condition (1) is satisfied, it becomes possible to satisfactorily correct chromatic aberration generated in the radial type gradient index lens element. If the lower limit of -0.05 of the condition (1) is exceeded, axial chromatic aberration will be overcorrected. On the other hand, when the value exceeds the upper limit of 0.01, axial chromatic aberration is insufficiently corrected, which is not preferable.
[0019]
Further, the lens system of the present invention makes it possible to satisfactorily correct the Petzval sum of the entire lens system by giving the homogeneous lens a negative refractive power. If the homogeneous lens has a positive refractive power, the Petzval sum of the entire lens system is insufficiently corrected, and the image plane is undesirably tilted to the object side.
[0020]
According to the second configuration of the present invention, there is provided a homogeneous lens having a negative refractive power and a radial refraction represented by the above-mentioned formula (a) in which a refractive index is distributed in a radial direction from an optical axis having a positive refractive power. The radial type gradient index lens element is composed of a refractive index distribution lens, and is characterized by satisfying the following condition (2):
[0021]
(2) 1 / V₁₀<1 / ν_h
Where ν_hIs the Abbe number of a homogeneous lens.
[0022]
By using two lenses, a relatively inexpensive imaging lens system can be achieved. However, in order to obtain a more inexpensive lens system, it is desirable that the radial type gradient index lens element has a biplanar shape. If the shape of the surface is flat, processing can be performed at extremely low cost as compared with the case of processing a spherical surface. Further, if the radial type gradient index lens is formed into a shape of both planes, the eccentricity between the surface of the radial type gradient index lens and the medium, which is a problem in actual processing, can be prevented, and assembly adjustment can be facilitated. .
[0023]
However, if the radial type gradient index lens is formed in a shape of both planes, the degree of freedom of aberration correction is reduced, and in particular, the Petzval sum may be insufficiently corrected.
[0024]
In the present invention, if the refractive power arrangement of the homogeneous lens and the radial type gradient index lens is set to an appropriate value, the Petzval sum can be satisfactorily corrected, and an imaging lens having excellent workability, inexpensiveness and high imaging performance can be obtained. I can do it.
[0025]
Next, aberration correction will be described in detail.
[0026]
In order to construct an imaging lens with two lenses, a homogeneous lens and a radial type gradient index lens on both planes, in order to obtain good optical performance, first consider Petzval sum and chromatic aberration among various aberrations. I have to. This is because the amount of generation of both aberrations is substantially determined by the refractive power arrangement of the lens system, and once determined, it becomes difficult to correct the lens due to bending of the lens.
[0027]
Here, the Petzval sum PTZ of the radial type gradient index lens is approximated by the following equation (e).
[0028]
PTZ = φ_S/ N₀₀+ Φ_m/ N₀₀ ²(E)
Also, the refractive power φ of the medium of the radial type gradient index lens element_mIs approximated by the following equation (f).
[0029]
φ_m≒ -2N₁₀t_G(F)
Where t_GIs the thickness of the radial type gradient index lens.
[0030]
In the imaging lens of the present invention, the radial type gradient index lens has a two-sided planar shape in order to improve manufacturability._S≒ 0, and Equations (d) and (e) have only the second term. Further, since the homogeneous lens is in close contact, the axial chromatic aberration and Petzval sum of the imaging lens of the present invention are represented by the following equations (g) and (h).
[0031]
PAC = K (φ_h/ Ν_h+ Φ_m/ V₁₀) (G)
PTZ = φ_h/ N_h+ Φ_m/ N₀₀ ²(H)
Where φ_hIs the refractive power of a thin homogeneous lens, ν_hIs the Abbe number of a homogeneous lens, n_hIs the refractive index of the homogeneous lens.
[0032]
From the above equations (g) and (h), the refractive power and V of the homogeneous lens and the radial type gradient index lens are calculated.₁₀It can be understood that desired values of the axial chromatic aberration and Petzval sum can be obtained by changing the parameters in the equations such as.
[0033]
In order to favorably correct axial chromatic aberration, the Abbe number is ν_hIn order to reduce axial chromatic aberration as compared with a homogeneous lens having the same refractive power φ as the imaging lens, it is necessary to satisfy the following expression.
[0034]
K (φ_h/ Ν_h+ Φ_m/ V₁₀) <K (φ / ν_h)
Where φ = φ_m+ Φ_h
Expanding the above equation, φ_m/ V₁₀<Φ_m/ Ν_hFrom this, the conditional expression (2) is derived.
[0035]
In other words, the Abbe number ν of a homogeneous lens_hNumber and medium Abbe number V of radial type gradient index lens₁₀Satisfies the relationship of the condition (2), it is possible to satisfactorily correct axial chromatic aberration. Condition (2) is a necessary condition for favorably correcting longitudinal chromatic aberration with the imaging lens of the present invention. If condition (2) is not satisfied, longitudinal chromatic aberration will be insufficiently corrected, which is not preferable.
[0036]
From equation (g), in order to favorably correct longitudinal chromatic aberration, it is necessary to set the refractive power arrangement of the homogeneous lens and the radial type gradient index lens to an appropriate value in addition to the condition (2). However, in setting the refractive power arrangement, it is necessary to sufficiently consider correction of Petzval sum in addition to chromatic aberration. This is because the amount of generation of the Petzval sum is substantially determined by the refractive power arrangement of the lens system as in the case of the chromatic aberration, as is apparent from the equation (h).
[0037]
Here, from the equations (g) and (h), the arrangement of the refractive power for simultaneously correcting the axial chromatic aberration of the imaging lens and the Petzval sum is considered. First, when the refractive power ratio a of the medium given by the following equation (i) is introduced into the equations (g) and (h), the equations (g ') and (h') are obtained, respectively.
[0038]
a = φ_m/ Φ (φ = φ_m+ Φ_h) (I)
PAC = Kφ ｛(ν_h-V₁₀) A + V₁₀｝ / (Ν_hV₁₀) (G ’)
PTZ = φ ｛(n_h-N₀₀ ²) A + N₀₀ ²｝ / (N_hN₀₀ ²) (H ')
Here, FIG. 20 is a graph of the equations (g ′) and (h ′) for easy understanding. In FIG. 20, the horizontal axis represents the refractive power ratio a of the medium, and the vertical axis represents the axial chromatic aberration PAC of the imaging lens and the Petzval sum PTZ. Also, a in FIG._PACIs the value of the refractive power ratio a of the medium when PAC = 0 in equation (g '), a_PTZIs the value of the refractive power ratio a of the medium when PTZ = 0 in the equation (h '), and these are given by the following equations (j) and (k).
[0039]
a_PAC= V₁₀/ (V₁₀−ν_h) (J)
a_PTZ= N₀₀ ²/ (N₀₀ ²-N_h) (K)
From FIG. 20, to simultaneously correct longitudinal chromatic aberration and Petzval sum, first a_PACAnd a_PTZNeed to have close values. For example, a_PACAnd a_PTZIf the signs are different, it is impossible to simultaneously correct the axial chromatic aberration and the Petzval sum with the imaging lens. In addition to this, if the value of the refractive power ratio a of the medium also has a value close to these values, it is possible to achieve an imaging lens in which the axial chromatic aberration and the Petzval sum are corrected extremely well.
[0040]
Here, the refractive index N on the optical axis in the production of the refractive index distribution material₀₀Does not differ significantly from the refractive index of existing homogeneous glass,₀₀Is about 1.5 to 1.9. Also, the refractive index n of the homogeneous glass_hIs also about 1.5 to 1.9, so that equation (k) satisfies the following condition (k ′).
[0041]
a_PTZ= N₀₀ ²/ (N₀₀ ²-N_h)> 1 '(k')
If the imaging lens of the present invention satisfies the condition (1), the expression (j) satisfies the following condition (j ′).
[0042]
a_PTC= V₁₀/ (V₁₀−ν_h)> 0 '(j')
From this, a when the axial chromatic aberration and the Petzval sum are favorably corrected,_PACAnd a_PTZIs 0 or more. Therefore, it is necessary that the refractive power ratio of the medium also becomes 0 or more.
[0043]
According to the relational expression (k '), it is more desirable that the value of the refractive power ratio a of the medium is 1 or more.
[0044]
That is, it is desirable that the refractive power of the medium of the radial type gradient index lens and the refractive power arrangement of the homogeneous lens have a relationship represented by the following equation (1).
[0045]
φ_m/ (Φ_m+ Φ_h)> 1 (l)
If the refractive power of the radial type gradient index lens and the homogeneous lens constituting the imaging lens satisfy Expression (1), it is possible to satisfactorily correct axial chromatic aberration and Petzval sum. If the condition (1) is not satisfied, it is difficult to simultaneously correct longitudinal chromatic aberration and Petzval sum.
[0046]
Further, since the imaging lens of the present invention has a positive refractive power, in order to satisfy the expression (1), φ_m> 0 and φ_hIt is necessary to satisfy <0. That is, it is necessary that the refractive power of the medium of the radial type gradient index lens has a positive value and the refractive power of the homogeneous lens has a negative value. If the homogeneous lens has a positive value, it becomes difficult to satisfactorily correct Petzval sum and axial chromatic aberration.
[0047]
In order to match the optical axes of the homogeneous lens and the radial type gradient index lens with high accuracy, at least a part or all of the surface of one side of the homogeneous lens has a planar shape, and the radial type refractive index It is desirable to adhere or adhere to the rate distribution lens. For example, by making the outer diameters of a homogeneous lens and a radial type gradient index lens equal, and adhering or adhering the two surfaces on a flat surface against the outer diameter, the optical axes of both lenses can be easily matched with high accuracy. It is possible. Another method is to measure the optical axis position of one or both of a homogeneous lens and a radial type gradient index lens, adjust the lens positions so that the optical axes of both lenses coincide, and bond or adhere both lenses. For example, the optical axes can be matched. At this time, the adjustment is extremely easy if both lenses have a flat portion and the two lenses are in contact with each other at this flat portion. In addition, there is an advantage that the lens frame structure can be simplified if both lenses are adhered or adhered. Further, even if a ring (spacer) or the like for adjusting the lens interval is inserted between the homogeneous lens and the radial type gradient index lens, substantially the same effects as described above can be obtained.
[0048]
Further, the third configuration of the present invention is represented by the above equation (a) in which the refractive index is distributed in the radial direction from the optical axis having the positive refractive power and the homogeneous lens having the negative refractive power in order from the object side. Focusing on a close-range object is performed by changing the distance between the homogeneous lens and the radial type gradient index lens.
[0049]
When the imaging lens of the present invention is applied to a video camera or the like that requires relatively high optical performance, it is necessary to sufficiently consider focusing. In other words, it is necessary that various aberrations from an object at infinity to an object at a close distance be properly corrected. In order to achieve further miniaturization, it is desirable that the burden on the drive mechanism for focusing be as small as possible.
[0050]
For the above reasons, in the imaging lens of the present invention, focusing is performed by changing the distance between the homogeneous lens and the radial type gradient index lens as described above. In other words, if focusing is performed using only one of the lenses, the load on the drive mechanism is extremely small because the movable lens is light, and the lens frame and the drive mechanism can be formed in a compact configuration. obtain. In order to reduce aberration fluctuation during focusing, it is possible to move the entire lens system to perform focusing, that is, so-called whole extension, but the weight of the movable lens increases and the load on the drive mechanism increases. However, it is not preferable because an imaging lens having a compact configuration cannot be achieved.
[0051]
Further, in order to reduce aberration fluctuations during focusing, it is desirable to perform focusing by moving only a homogeneous lens having a negative refractive power. Further, it is desirable that a negative homogeneous lens and a positive radial type gradient index lens are arranged in order from the object side, and the negative first lens is extended toward the object side to perform focusing on a close object. When the first lens is moved toward the object side to perform focusing on a close object, the fluctuation of the ray height of the axially incident light beam is extremely small, so that the fluctuation of the spherical aberration particularly during focusing can be reduced. Further, since the amount of generation of the axial chromatic aberration and the Petzval sum does not change so much, it is possible to obtain good imaging performance.
[0052]
Further, as described above, in order to easily match the optical axis of the homogeneous lens with the optical axis of the radial type gradient index lens element with high accuracy, a part or all of at least one surface of the homogeneous lens is formed into a planar shape. It is desirable to adhere or adhere to the radial type gradient index lens directly or via an interval ring or the like at the portion. However, when focusing is performed by changing the distance between the homogeneous lens and the radial type gradient index lens, the two lenses are once brought into close contact with each other directly or via an interval ring at the time of assembling, and the optical axis positions coincide at that time. It is desirable to adjust the positions of both lenses as described above.
[0053]
Further, the lens system of the present invention can be applied to an imaging system using an image guide composed of a solid-state imaging device such as a CCD or a fiber bundle, but in that case, in order to secure a sufficient amount of peripheral light, in order from the object side. , A first lens having a negative refractive power and a second lens having a positive refractive power. With a negative-positive configuration in order from the object side, the angle of incidence of off-axis rays on the image plane can be made closer to vertical. In other words, it can be made closer to parallel to the optical axis, and sufficient peripheral light quantity can be secured. If the configuration is positive-negative in order from the object side, it is difficult to make the incident angle of the off-axis ray on the image plane close to vertical, and it is difficult to secure a sufficient amount of peripheral light.
[0054]
In the imaging lens of the present invention, in order to favorably correct axial chromatic aberration and Petzval's sum, it is desirable that the homogeneous lens has a somewhat large negative refractive power. As described above, the refractive power of the medium of the radial type gradient index lens has a positive value, and the refractive power of the homogeneous lens has a negative value, which is necessary for satisfactorily correcting axial chromatic aberration and Petzval sum. Condition. In this case, if the refractive power of the homogeneous lens is as small as possible, it becomes difficult to satisfactorily correct the Petzval sum in particular.
[0055]
For the above reasons, it is desirable that the lens system of the present invention satisfies the following condition (3).
[0056]
(3) −1.5 <f / f_h<-0.05
Where f is the focal length of the entire photographing lens system, f_hIs the focal length of the homogeneous lens.
[0057]
If the imaging lens of the present invention satisfies the above condition (3), it becomes possible to favorably correct the Petzval sum. If the value exceeds the upper limit of -0.05 of the condition (3), the Petzval sum is insufficiently corrected, and the image plane is tilted to the object side, which is not preferable. On the other hand, if the value exceeds the lower limit of -1.5, the Petzval sum becomes excessively corrected, and the image plane is tilted away from the object, which is not preferable.
[0058]
When the first lens is composed of a first lens of a negative lens and a second lens of a positive lens in order from the object side and the first lens is used for focusing, if the first lens is extended to the object side, off-axis aberrations, particularly Although the distortion tends to be large, the distortion can be satisfactorily corrected by setting the negative refractive power of the first lens to an appropriate value. Therefore, when the first lens is used for focusing in the imaging lens of the present invention, it is desirable that the above condition (3) is satisfied.
[0059]
If the focal length of the first lens satisfies the condition (3), it is possible to reduce the fluctuation of off-axis aberration due to focusing. If the lower limit of -1.5 of the condition (3) is exceeded, the negative refractive power of the first lens becomes too large, and the off-axis aberration is deteriorated. On the other hand, when the value exceeds the upper limit of -0.05, the negative refractive power becomes weak, the moving distance for focusing becomes long, and it becomes difficult to reduce the size of the lens system.
[0060]
Further, since the imaging lens of the present invention has a positive refractive power, it tends to generate negative spherical aberration in the whole system. However, the imaging lens is constituted by a homogeneous lens having a negative refractive power and a radial type gradient index lens on both planes. As a result, it is possible to make a good correction. For example, if a radial type gradient index lens having a positive refractive power is provided with a concave surface instead of both flat surfaces, the refractive index of the concave surface decreases from the optical axis to the periphery, so the amount of positive spherical aberration generated Has the effect of reducing Therefore, in such an imaging lens, the negative spherical aberration is insufficiently corrected.
[0061]
However, since the radial type gradient index lens used in the imaging lens of the present invention has a planar shape, no spherical aberration occurs on the surface, and a positive spherical aberration occurs on a homogeneous lens having a negative refractive power. In this case, it becomes possible to favorably correct spherical aberration.
[0062]
As described above, the imaging lens of the present invention is excellent in manufacturability, and the axial chromatic aberration and Petzval's sum are satisfactorily corrected, and in addition, the spherical aberration can be satisfactorily corrected.
[0063]
When the imaging lens of the present invention is applied to a video camera or the like, it is desired that the angle of view be wide to some extent. In the imaging lens of the present invention, by adding a negative lens to the object side of the radial type gradient index lens, it becomes possible to widen the angle of view. In addition, it is desirable to increase the refractive power of the radial type gradient index lens to some extent. If the refractive power of the radial type gradient index lens element is large, a lens system having a short focal length of the entire system and a wide angle of view can be achieved.
[0064]
Here, it is considered to increase the refractive power of the medium of the radial type gradient index lens. From the above equation (f), to increase the refractive power of the medium, N₁₀Or t_GIt is necessary to increase. However, in the production of the refractive index distribution material, N₁₀It is not preferable since it is difficult to produce a material having an extremely large Therefore, in the present invention, N₁₀It was considered that the radial type gradient index lens should have an appropriate lens thickness so as to obtain a sufficient refractive power without excessively increasing the refractive index. That is, in the lens system of the present invention, it is desirable that the radial type gradient index lens satisfy the following condition (4).
[0065]
(4) $ 0.5 <t_G/ F <9
Here, f is the focal length of the entire imaging lens system.
[0066]
If the condition (4) is satisfied, it is possible to effectively use the effect of correcting the axial chromatic aberration and the Petzval sum. If the lower limit of 0.5 of the condition (4) is exceeded, the refractive power of the radial type gradient index lens element will be weak, and it will be difficult to effectively use the correction effects of axial chromatic aberration and Petzval sum. On the other hand, if the value exceeds the upper limit of 9, the thickness of the lens becomes too large and an image is formed inside the lens, which is not preferable.
[0067]
Further, when the imaging lens of the present invention is applied to an imaging system that requires finer pixel pitch of a solid-state imaging device and requires higher performance imaging performance, it is preferable that the following condition (5) is further satisfied.
[0068]
(5) 1.5 <t_G/ F <7
If the lower limit of 1.5 of the condition (5) is exceeded, the refractive power of the imaging lens becomes weak, and the effect of correcting axial chromatic aberration and Petzval sum is effectively used in a lens system requiring high-performance imaging performance. It becomes difficult. On the other hand, when the value exceeds the upper limit of 7, the lens becomes thick and the transmittance and the flare deteriorate, so that it is difficult to achieve high-performance imaging performance.
[0069]
Further, when the imaging lens of the present invention is applied to a lens system requiring higher imaging performance, it is desirable that the following condition (6) is satisfied.
[0070]
(6) Δ−0.01 <1 / V₁₀<0.008
If the radial type gradient index lens satisfies the condition (6), it may be possible to correct axial chromatic aberration more favorably. If the value exceeds the upper limit of 0.008 of the condition (6), the axial chromatic aberration is insufficiently corrected, which is not preferable. On the other hand, when the value exceeds the lower limit of -0.01, axial chromatic aberration is excessively corrected, which is not preferable.
[0071]
The radial type gradient index lens has a fourth order term N of the refractive index distribution.₂₀It is possible to control the amount of spherical aberration by changing the value of. In order to favorably correct spherical aberration in the imaging lens of the present invention, it is desirable to satisfy the following condition (7).
[0072]
(7) −0.2 <N₂₀× f⁴<0.2
If the radial type gradient index lens satisfies the condition (7), it is possible to satisfactorily correct spherical aberration. If the value exceeds the upper limit of 0.2 of the condition (7), the spherical aberration is excessively corrected, which is not preferable. If the lower limit of -0.2 is exceeded, the spherical aberration will be insufficiently corrected, which is not preferable.
[0073]
Further, when the imaging lens of the present invention is applied to a lens system requiring higher imaging performance, it is desirable that the following condition (8) is satisfied.
[0074]
(8) -0.05 <N₂₀× f⁴<0.05
If the radial type gradient index lens satisfies the condition (8), it becomes possible to correct spherical aberration more favorably. If the value exceeds the upper limit of 0.05 of the condition (8), spherical aberration is excessively corrected, and high-performance imaging performance cannot be achieved. On the other hand, when the value exceeds the lower limit of -0.05, spherical aberration is insufficiently corrected, and high-performance imaging performance cannot be achieved.
[0075]
Further, when the imaging lens of the present invention satisfactorily corrects not only longitudinal chromatic aberration but also lateral chromatic aberration, it is desirable to satisfy the following condition (9).
[0076]
(9) 1 / ν_h<0.03
Since the imaging lens of the present invention has different ray heights of the off-axis chief ray passing through the homogeneous lens and the radial type gradient index lens, any of the lenses can be used to correct chromatic aberration of magnification in addition to axial chromatic aberration. It is desirable that the amount of chromatic aberration be small. For the radial type gradient index lens element, the amount of chromatic aberration can be reduced by satisfying the condition (2). Therefore, it is desirable that the homogeneous lens satisfies the condition (9). If the condition (9) is satisfied, it is possible to reduce the chromatic aberration of magnification in addition to the axial chromatic aberration. If the condition (9) is not satisfied, it becomes difficult to satisfactorily correct lateral chromatic aberration.
[0077]
Further, when the imaging lens of the present invention is applied to a lens system requiring higher imaging performance, it is desirable that the following condition (10) is satisfied.
[0078]
(10) 1 / ν_h<0.025
If the condition (10) is satisfied, chromatic aberration of magnification can be corrected more favorably. If the condition (10) is not satisfied, chromatic aberration of magnification is insufficiently corrected, and high-performance imaging performance cannot be achieved.
[0079]
For better correction of Petzval sum in the imaging lens system of the present invention, it is desirable to satisfy the following condition (11).
[0080]
(11) N_00d> 1.55
If the condition (11) is satisfied, the value of the second term of the expression (h) becomes smaller, and the Petzval sum generated in the medium of the radial type gradient index lens can be sufficiently reduced. If the condition (11) is not satisfied, the Petzval sum generated in the medium is insufficiently corrected, which is not preferable.
[0081]
Further, when the lens system of the present invention is applied to a lens system having a relatively wide angle of view, it is necessary to increase the refractive power φ of the entire lens system. In addition, in order to favorably correct longitudinal chromatic aberration, it is desirable to satisfy the following condition (12).
[0082]
(12) 0.5 <a_PAC<1.7
Refractive power of the whole lens system φ (= φ_m+ Φ_h), The refractive power ratio a of the medium given by equation (i) has a value close to 1. In addition, as described above, in order to favorably correct longitudinal chromatic aberration, a_PACIs preferably close to the value of the refractive power a of the medium. Therefore, it is desirable that the lens system of the present invention satisfies the condition (12). That is, if the condition (12) is satisfied, it is possible to satisfactorily correct longitudinal chromatic aberration. If the value exceeds the upper limit of 1.7 of the condition (12), the axial chromatic aberration is insufficiently corrected. If the value exceeds the lower limit of 0.5, the correction becomes excessive, which is not preferable.
[0083]
Further, in the imaging lens system of the present invention, the homogeneous lens or the radial type gradient index lens is an optical element having an effect such as a low-pass filter, an infrared cut filter, or a band cut filter for cutting a specific wavelength component. It is also possible to realize a high-performance imaging lens.
[0084]
In addition, by making the homogeneous spherical lens an aspherical surface, various aberrations can be corrected more favorably.
[0085]
If the load on the driving mechanism for focusing does not cause much problem, it is also possible to perform so-called whole feeding, in which the homogeneous lens and the radial type gradient index lens are moved integrally.
[0086]
BEST MODE FOR CARRYING OUT THE INVENTION
Next, embodiments of the present invention will be described based on the following examples.

[0087]

[0088]

[0089]

[0090]

[0091]

[0092]

[0093]

Where r₁, R₂,... Are the radius of curvature of each surface of the lens, d₁, D₂,... Indicate the thickness and lens spacing of each lens, n₁, N₂,... Are the refractive indices of the d-line of each lens, ν₁, Ν₂,... Are Abbe numbers of the respective lenses.
[0094]
Embodiment 1 of the present invention is shown in FIG. In other words, the imaging lens is a two-lens configuration of a first lens of a negative lens and a second lens of a positive lens, and the second lens is formed of a radial type gradient index lens on both planes in order from the object side. The negative first lens has a concave surface on the object side and a flat surface on the image side, and the flat portion is in close contact with the radial type gradient index lens. In addition, the provision of the stop S between the homogeneous lens and the radial type gradient index lens element helps to reduce the height of off-axis rays passing through the lens system and to reduce the diameter of the lens.
[0095]
By arranging a solid-state imaging device such as a CCD at the image plane position, the imaging lens of this embodiment can be used as an imaging system used for a video camera, a videophone, or an intercom, for example. A filter F for cutting a specific wavelength component is adhered or adhered to the image-side surface of the radial type gradient index lens. Further, if a radial type gradient index lens or a homogeneous lens is provided with a filter function for cutting a specific wavelength component, the filter F in FIG. 1 can be omitted. Further, the filter can be arranged separately from the radial type gradient index lens.
[0096]
In the first embodiment, focusing on an object at a close distance is performed by extending the first lens toward the object side. Therefore, a configuration is adopted in which the homogeneous lens and the radial type gradient index lens adhere to each other without bonding.
[0097]
In focusing on a short-distance object (object point position 100 mm) in this embodiment, the distance between the first lens and the second lens is 3.810 mm, and the stop is in close contact with the second lens and does not move.
[0098]
In the first embodiment, since the outer diameters of the homogeneous lens and the radial type gradient index lens are equal, the optical axes of both lenses can be easily matched with relatively high accuracy.
[0099]
When focusing is not performed in the lens system of the present invention, a homogeneous lens and a radial type heterogeneous lens can be bonded for simplification of a lens frame or the like.
[0100]
In the first embodiment, since the homogeneous lens has a negative refractive power, the 1 / V of the radial type gradient index lens is used to favorably correct axial chromatic aberration.₁₀Has a small positive value.
[0101]
Considering that the imaging lens of the present invention is manufactured at low cost, it is desirable that the homogeneous lens has a concave surface and a planar shape as in the first embodiment. If one side has a planar shape, polishing is extremely easy and processing can be performed at low cost.
[0102]
Further, the imaging lens of the present invention has an advantage of aberration correction which cannot be obtained by a radial type gradient index lens which is directly spherically processed. FIG. 21 shows the refractive index n on the vertical axis._d, The horizontal axis is Abbe number ν_dThe area surrounded by the thick line is the approximate existing range of the existing glass, the arrow is the change in the refractive index and Abbe number of the radial type gradient index lens, and the black circle is the refractive index and Abbe number on the optical axis. Is represented. For example, the values of the refractive index and Abbe number distributed from the optical axis to the periphery of the radial type gradient index lens material produced by the ion exchange method or the sol-gel method are indicated by arrows A in FIG. As shown, it is almost within the range of existing glass. Therefore, for example, as shown by an arrow B in FIG. 21, the refractive index on the optical axis and the Abbe number are within the range of the existing glass, and the refractive index and the Abbe number deviate from the range of the existing glass toward the periphery. It is extremely difficult to produce such a radial type gradient index lens material. However, such a material may be desired in correcting various aberrations. For example, a material represented by an arrow B has a small variance and is therefore a material advantageous for correcting chromatic aberration. However, if the imaging lens of the present invention is used, even if the radial type gradient index lens is a material within the range of the existing glass, it is almost the same as a material having a refractive index distribution that deviates from the range of the existing glass described above. It is possible to obtain the effect of This will be specifically described using the first embodiment. An arrow A indicates the radial type gradient index lens element according to the first embodiment.₀₀= 1.75, V₀₀= 45, V₁₀≒ 200 axial chromatic aberration PAC_AIs represented by the following equation.
[0103]
PAC_A= K (φ_s/ 45 + φ_m/ 200)
Also, the axial chromatic aberration PAC of the arrow B_BIs represented by the following equation.
[0104]
PAC_B= K (φ_s/70.21+φ_m/ 200)
However, in the first embodiment, n_h= 1.48749, ν_h= 70.21 and a radial type gradient index lens, and the axial chromatic aberration PAC of this combined lens_CBecomes the following equation.
[0105]
PAC_C= K (φ_h/70.21+φ_m/ 200)
Therefore, φ_h≒ φ_sIf the refractive power of the homogeneous lens is selected so as to satisfy the following condition, it is possible to achieve an imaging lens having a chromatic aberration correction effect substantially similar to that of the material represented by the arrow B which is difficult to manufacture in practice. As described above, the imaging lens of the present invention has an advantage that the degree of freedom of aberration correction is increased by selecting a desired homogeneous lens.
[0106]
The aberration states of the imaging lens of the first embodiment are as shown in FIGS. 9 and 10, where FIG. 9 is for an object point at infinity and FIG. 10 is for an object point at a short distance. As can be seen from these aberration diagrams, various aberrations are favorably corrected despite the fact that the number of lenses is two.
[0107]
Embodiment 2 of the present invention is shown in FIG. In other words, the imaging lens is a two-lens configuration of a first lens of a negative lens and a second lens of a positive lens, and the second lens is formed of a radial type gradient index lens on both planes in order from the object side. The first lens has a meniscus shape with the concave surface facing the image side, has a flat portion 1 on the outer periphery than the effective diameter of the surface on the image surface side, and the flat portion 1 has a radial type gradient index lens. Adhered or adhered. A filter F for cutting a specific wavelength component is used on the image side of the radial type gradient index lens.
[0108]
In the second embodiment, the homogeneous lens of the imaging lens used in the first embodiment is replaced with a meniscus-shaped homogeneous lens. The imaging lens having a different focal length is used despite the fact that the radial type gradient index lens is common. This is a realized example. That is, as shown in FIG. 22, the imaging lens of the present invention has a radial type gradient index lens L._g, And a uniform lens of various shapes having different r, d, and n, for example, L_h1, L_h2, L_h3Are combined, the imaging lens L having a different focal length and different amounts of various aberrations despite the fact that there is only one type of radial type gradient index lens._c1, L_c2, L_c3Can be achieved. As described above, since the radial type gradient index lens can be used as a common component, it is possible to realize an inexpensive lens system in spite of supporting various lens systems used under a white light source.
[0109]
Further, by providing the stop s on the object side of the homogeneous lens, particularly off-axis aberrations can be favorably corrected.
[0110]
In this embodiment, since the focal length is relatively long and it is particularly difficult to correct axial chromatic aberration, the Abbe number is 1 / ν_hHighly dispersed glass satisfying> 0.03 was used. In addition, since the stop is provided closest to the object side, it is desirable to use high dispersion glass for the homogeneous lens in order to satisfactorily correct lateral chromatic aberration.
[0111]
In addition, various aberrations are favorably corrected by using an aspherical surface on the image surface side of the homogeneous lens. The aspherical shape used in the examples is represented by the following equation.

[0112]
Here, the above equation takes the x axis in the optical axis direction and the y axis in the direction perpendicular to the optical axis, where r is the radius of curvature on the optical axis, P is the conic constant, A_2iIs an aspheric coefficient.
[0113]
In the second embodiment as well, focusing is performed by moving the first lens to the object side. At the time of focusing on a short-distance object (object point distance 100 mm) according to the second embodiment, the distance between the first lens and the second lens is 1.5151 mm, and the diaphragm moves together with the first lens.
[0114]
The aberration states of the second embodiment are as shown in FIGS. 11 and 12, where FIG. 11 is for an object point at infinity and FIG. 12 is for an object point at a short distance. It can be seen that in the imaging lens of Example 2, various aberrations are satisfactorily corrected despite the fact that the number of lenses is two.
[0115]
Embodiment 3 of the present invention is shown in FIG. In other words, the imaging lens is a two-lens configuration of a first lens of a negative lens and a second lens of a positive lens, and the second lens is formed of a radial type gradient index lens on both planes in order from the object side. The first lens is a plano-concave lens, and has a flat portion 1 on an outer peripheral portion of the effective diameter of the image-side surface, and the flat portion 1 is adhered or adhered to the radial type gradient index lens. The stop S is provided at a position of 1 mm from the object side surface of the radial type gradient index lens toward the image side. Further, a filter F for cutting a specific wavelength component is used on the image side of the radial type gradient index lens.
[0116]
Embodiment 3 is an example in which the lens system is widened by increasing the negative refractive power of the homogeneous lens, and can be used for, for example, an objective lens of an endoscope. When the imaging lens of the present invention is applied to a lens having a wide angle of view as in Embodiment 3, it is desirable that the following condition (13) is satisfied.
[0117]
(13) −1.5 <f / f_h<-0.4
If the upper limit of -0.4 of the condition (13) is exceeded, it is difficult to achieve a wide angle of view. On the other hand, if the value exceeds the lower limit of -1.5, the Petzval sum is excessively corrected, and the image plane falls in the direction away from the object, which is not preferable.
[0118]
In this embodiment, since the lens system has a wide angle of view, it is particularly difficult to correct the Petzval sum._00dThis is well corrected by satisfying> 1.6. Where N_00dSatisfaction of> 1.65 is more preferable because the Petzval sum can be corrected more favorably.
[0119]
In addition, by providing the diaphragm inside the radial type gradient index lens element, in addition to reducing the diameter of the lens, it has become possible to favorably correct particularly off-axis aberrations.
[0120]
When the homogeneous lens of the imaging lens of the present invention and the radial type gradient index lens are bonded together, an adhesive may be applied to a flat portion indicated by 1 in FIG. 3 and assembled, but reference numeral B in FIG. It is also possible to apply an adhesive to the outer peripheral portion of the lens and bond the portion indicated by the symbol as indicated by 4 in FIG.
[0121]
The aberration state of the third embodiment is as shown in FIG. It can be seen that in the imaging lens of Example 3, various aberrations are satisfactorily corrected despite the fact that the number of lenses is two.
[0122]
Embodiment 4 of the present invention is shown in FIG. In other words, the imaging lens is a two-lens configuration of a first lens of a negative lens and a second lens of a positive lens, and the second lens is formed of a radial type gradient index lens on both planes in order from the object side. The first lens is a concave flat lens, and is adhered or adhered to the radial type gradient index lens on the image plane side. Further, the solid-state imaging device or the solid-state imaging device unit C is adhered or adhered to the image side portion of the radial type gradient index lens element, which is effective for miniaturization or simplification of the lens frame configuration.
[0123]
Further, when the imaging lens of the present invention is applied to a lens system having a relatively narrow angle of view, it is not preferable that a homogeneous lens having a negative refractive power has an extremely large value, and the following condition (14) is satisfied. It is desirable.
[0124]
(14) −0.8 <f / f_h<-0.1
If the lower limit of the condition (14), i.e., -0.8, is exceeded, the Petzval sum will be overcorrected, and the image plane will fall in the direction away from the object. On the other hand, if the value exceeds the upper limit of -0.1, the Petzval sum is insufficiently corrected and the image plane is tilted toward the object side, which is not preferable.
[0125]
In addition, when the imaging lens of the present invention is applied to an optical system in which transmittance and flare pose a problem, it is desirable that the total length of the radial type gradient index lens element be about 40 mm or less. Further, as in the present embodiment, if the radial type gradient index lens is about 25 mm or less, a lens system having better imaging performance can be achieved.
[0126]
Further, in order to prevent flare on the outer peripheral side surface of the lens, it is effective to make the outer peripheral side surface of the lens mat-like, or to paint a relatively dark color paint such as black on the outer peripheral side surface of the lens.
[0127]
The aberration state of the fourth embodiment is as shown in FIG. As shown in this drawing, it can be seen that the aberrations of the imaging lens of Example 4 are favorably corrected in spite of the fact that the number of lenses is two.
[0128]
The fifth embodiment has the configuration shown in FIG. In other words, the imaging lens is a two-lens configuration of a first lens of a negative lens and a second lens of a positive lens, and the second lens is formed of a radial type gradient index lens on both planes in order from the object side. The first lens is a biconcave lens, and is adhered or adhered to the radial type gradient index lens at the plane portion 1 on the image plane side. A brightness stop S is provided at a position 5 mm from the object side surface of the radial type gradient index lens to the image surface side. Further, a field stop FS for cutting a flare component or the like is provided in the radial type gradient index lens. These diaphragms are cut from the outer periphery of the radial type gradient index lens, and paint is applied to the cut surface to block the light beam, or the radial type gradient index lens is cut to stop at the cut surface. Can be produced by printing, vapor deposition or sandwiching a thin plate and bonding or adhering again.
[0129]
The aberration state of the fifth embodiment is as shown in FIG. As shown in this drawing, it can be seen that the aberrations of the imaging lens of Example 5 are favorably corrected despite the fact that the number of lenses is two.
[0130]
Embodiment 6 of the present invention is shown in FIG. In other words, the imaging lens is a two-lens configuration of a first lens of a negative lens and a second lens of a positive lens, and the second lens is formed of a radial type gradient index lens on both planes in order from the object side. The first lens is a concave flat lens, and is adhered or adhered to the radial type gradient index lens on the surface on the image side.
[0131]
Further, the diameter of the radial type gradient index lens is made different from the diameter of the homogeneous lens, and the lens frame or a part 6 of the lens frame can be efficiently arranged, thereby achieving the miniaturization of the lens system.
[0132]
The aberration state of the sixth embodiment is as shown in FIG. As can be seen from the drawing, various aberrations are satisfactorily corrected in the imaging lens of the sixth embodiment in spite of the fact that the number of lenses is two.
[0133]
Embodiment 7 of the present invention is shown in FIG. In other words, in order from the object side, it is composed of a first lens of a radial type heterogeneous lens having a positive refractive power and a second lens of a homogeneous lens having a negative refractive power. The homogeneous lens of the second lens has a concave flat shape and has a flat portion 1 on the outer peripheral portion of the effective diameter of the object side surface, and the flat portion is adhered or adhered to the radial heterogeneous lens. The stop S is provided at a position of 2.7369 mm closer to the image side than the surface of the radial type heterogeneous lens on the object side.
[0134]
The seventh embodiment is an optical system for forming an image at the same magnification, and can be used as an imaging lens system such as a video microscope if a solid-state imaging device is arranged at an image plane position.
[0135]
The aberration state of the seventh embodiment is as shown in FIG. As shown in this figure, it can be seen that the aberrations of the imaging lens of Example 7 are satisfactorily corrected even though the number of lenses is two.
[0136]
The eighth embodiment has the configuration shown in FIG. In other words, the imaging lens is a two-lens configuration of a first lens of a negative lens and a second lens of a positive lens, and the second lens is formed of a radial type gradient index lens on both planes in order from the object side. The first lens is a biconcave lens, and the spacing ring 7 is sandwiched between the plane part 1 on the image plane side and the plane part 8 of the radial type gradient index lens. The aperture stop S is provided at a distance of 3.8973 mm from the object-side surface of the radial type gradient index lens. When focusing is performed by changing the distance between the first lens and the second lens, it is desirable to dispose the first lens, the distance ring 7, and the radial type gradient index lens in FIG. 8B without bonding. It is also possible to bond the first lens and the spacing ring 7 or the spacing ring 7 and the radial type gradient index lens. In the eighth embodiment, the spacing ring 7 is a thin plate having two flat surfaces. For example, the spacing ring 7 may have a surface oblique to the optical axis as shown by a hatched portion in FIG. It is also possible to In the case of FIG. 23, the plane portion of the spacing ring 7 and the radial type gradient index lens L_gBy bringing the flat surface portion 6 into close contact and sliding the close contact surface, adjustment can be made so that the optical axis positions of both lenses coincide. Further, as shown in FIG. 24, it is also possible to make the relatively sharp portion 8 of the homogeneous concave lens closely adhere to the radial type gradient index lens, and to adjust the optical axes of both lenses without interposing the spacing ring. It is.
[0137]
In order to achieve higher imaging performance, it is desirable that the homogeneous lens has a biconcave shape as in this embodiment. In the case of a biconcave shape, a negative refractive power can be distributed to both surfaces, so that various aberrations generated by this lens can be reduced.
[0138]
In the eighth embodiment, the distance between the first lens and the second lens at the time of focusing on a short-distance object (object point distance 100 mm) is 0.6148 mm, and the aperture is fixed in the second lens (refractive index distribution lens). ing.
[0139]
The aberration state of the eighth embodiment is as shown in FIG. As shown in this figure, it can be seen that the aberrations of the imaging lens of Example 8 are satisfactorily corrected even though the number of lenses is two.
[0140]
Next, a case where the imaging lens of the present invention is manufactured will be described in detail with reference to FIGS.
[0141]
FIG. 25 shows a conventional example, which is a model in which spherical processing is performed directly on a radial type gradient index lens. FIG. 25A is a diagram showing a cylindrical radial refractive index distribution lens and a refractive index distribution before spherical processing, and FIG._gRepresents a radial type gradient index lens, n (r) represents a refractive index at a point of radius r, and the optical axis 10 of the medium coincides with the central axis of the outer peripheral portion 13. FIG. 25B is an ideal example when one surface 14 is processed into a spherical surface, and the optical axis of the medium coincides with the optical axis of the medium. However, in an actual processing scene, as shown in FIG.₁Is difficult to process easily with high precision, and as shown in FIG. 25C, the optical axis 10 of the medium and the surface R₁The eccentricity of δ and the inclination angle ε may occur in the direction 16 perpendicular to the optical axis of the medium. In the case of a homogeneous lens, even if such an eccentricity occurs, the eccentricity does not matter because the centering process for cutting the outer periphery of the lens against the optical axis of the polished surface is not a problem. Since the optical axis is decentered with respect to the outer peripheral portion, centering cannot be performed.
[0142]
Therefore, in the lens system of the present invention, as shown in a model in FIG. 26, a radial type refractive index distribution lens having a shape of both planes and a homogeneous spherical lens having a plane portion 19 are adhered or adhered. FIG. 26A shows a radial type gradient index lens L having both surfaces flattened._gAnd a homogeneous lens L having a flat portion 19 on one side_hIs represented. Radial type gradient index lens L_gThe optical axis 10 of the medium is coincident with the central axis of the outer peripheral portion 13 and the homogeneous lens L_hIs a surface R because centering is possible.₁The optical axis 16 can easily coincide with the central axis of the outer peripheral portion 21 with high accuracy. FIG. 26B shows a radial type gradient index lens L._gAnd homogeneous lens L_hAre adhered or adhered to the flat portions 20 and 19 respectively. At this time, for example, the outer peripheral portion 13 of the radial type gradient index lens and the homogeneous lens L_hIf the outer peripheral portions 21 of the lenses coincide with each other, the optical axis of the medium of the radial type gradient index lens and the optical axis of the homogeneous lens can be easily matched.
[0143]
In the conventional method shown in FIG. 25, when eccentricity occurs during spherical processing, the radial type gradient index lens L_gHowever, in the method shown in FIG. 26 of the present invention, as described above, eccentricity at the time of spherical machining does not matter at all, so that the yield can be increased and the machining can be performed extremely inexpensively. It is possible. Further, since both surfaces of the radial type gradient index lens are flat, they can be processed with high precision and at low cost.
[0144]
FIG. 27 shows an example in which the imaging lens of the present invention is incorporated in a mirror frame having a very simple structure. L in FIG._hIs a concave lens, L_gIs a radial type gradient index lens, and 25 is a mirror frame, which in this case has a cylindrical shape. Reference numerals 26 and 27 denote a solid-state image sensor and its processing circuit, and reference numeral 28 denotes a cover glass for protecting the lens. With the configuration as shown in FIG. 27, since a homogeneous lens and a radial type gradient index lens can be dropped into the lens frame 25, assembly is extremely easy. When the imaging lens is held by the lens frame as in the present embodiment, the homogeneous lens and the radial type gradient index lens need not necessarily be bonded. In addition, if a homogeneous lens and a radial type gradient index lens are bonded together in advance and then assembled into a lens frame, assembly can be performed with high accuracy. At this time, it is also possible to attach the image pickup element to the radial type gradient index lens in advance.
[0145]
Further, the cover glass 28 can be formed of an optical filter for cutting a specific wavelength component.
[0146]
FIG. 28 shows the structure of a lens frame for performing focusing with the imaging lens of the present invention. L in FIG. 28_hIs a concave lens, L_gIs a radial type gradient index lens, 25 is a mirror frame, and in this case, a radial type gradient index lens L_gHolding. Reference numerals 26 and 27 denote a solid-state image sensor and its processing circuit unit, and 29 denotes a mirror frame which is a homogeneous lens L._hHolding. As shown in FIG._hAnd radial type gradient index lens L_gAre held by different lens frame parts 25 and 29, so that focusing can be performed. In FIG. 28, the lens frame 25 and the lens frame 29 are normally connected via a cam groove as used in a lens frame of a silver halide camera or the like. May have a smooth shape, and one of the lens frames may slide to perform focusing.
[0147]
The imaging lens described in each of the following sections can also achieve the object of the present invention.
[0148]
(1) An imaging lens that satisfies the following condition (3), which is the lens system according to claim 1, 2, or 3.
[0149]
(3) −1.5 <f / f_f<-0.05
(2) An imaging lens which satisfies the following condition (4), which is the lens system described in claim 1, 2, or 3, or in the above item (1).
[0150]
(4) $ 0.5 <t_G/ F <9
(3) An imaging lens which satisfies the following condition (5), which is the lens system described in claim 1, 2, or 3, or (1) or (2).
[0151]
(5) 1.5 <t_G/ F <7
(4) An imaging lens which satisfies the following condition (6), which is the lens system described in claim 2 or 3 of the claims or in the above item (1), (2) or (3).
[0152]
(6) Δ−0.01 <1 / V₁₀<0.008
(5) In the lens system described in claim 1, 2, or 3, or in the above item (1), (2), (3) or (4), the following condition (7) is satisfied. A satisfying imaging lens.
[0153]
(7) −0.2 <N₂₀× f⁴<0.2
(6) In the lens system described in claim 1, 2, or 3, or in the above item (1), (2), (3), or (4), the following condition (8) is satisfied. A satisfying imaging lens.
[0154]
(8) -0.05 <N₂₀× f⁴<0.05
(7) The lens system according to claim 1, 2, or 3, or the lens system described in (1), (2), (3), (4), (5), or (6). The imaging lens satisfying the following condition (9):
[0155]
(9) 1 / ν_h<0.03
(8) A lens system according to claim 1, 2, or 3, or (1), (2), (3), (4), (5) or (6), An imaging lens satisfying the following condition (10).
[0156]
(10) 1 / ν_h<0.025
(9) According to Claims 1, 2 or 3 of the claims or the above (1), (2), (3), (4), (5), (6), (7) or (8) An imaging lens which satisfies the following condition (11) with the described lens system.
[0157]
(11) N_00d> 1.55
(10) Claims 1, 2 or 3 in the claims or (1), (2), (3), (4), (5), (6), (7), (8) or An imaging lens which satisfies the following condition (12) in the lens system described in (9).
[0158]
(12) 0.5 <a_PAC<1.7
(11) Claims 1, 2 or 3 of the claims or the above (1), (2), (3), (4), (5), (6), (7), (8), An imaging lens which satisfies the following condition (13) in the lens system described in (9) or (10).
[0159]
(13) −1.5 <f / f_h<-0.4
(12) Claims 1, 2, or 3 of the claims or the above (1), (2), (3), (4), (5), (6), (7), (8), An imaging lens which satisfies the following condition (14) in the lens system described in (9) or (10).
[0160]
(14) −0.8 <f / f_h<-0.1
[0161]
【The invention's effect】
The imaging lens of the present invention is excellent in workability, and various aberrations, particularly, longitudinal chromatic aberration and Petzval sum are satisfactorily corrected.
[Brief description of the drawings]
FIG. 1 is a sectional view of a first embodiment of the present invention.
FIG. 2 is a sectional view of a second embodiment of the present invention.
FIG. 3 is a sectional view of a third embodiment of the present invention.
FIG. 4 is a sectional view of a fourth embodiment of the present invention.
FIG. 5 is a sectional view of a fifth embodiment of the present invention.
FIG. 6 is a sectional view of a sixth embodiment of the present invention.
FIG. 7 is a sectional view of a seventh embodiment of the present invention.
FIG. 8 is a sectional view of an eighth embodiment of the present invention.
FIG. 9 is an aberration curve diagram for an object point at infinity according to the first embodiment of the present invention.
FIG. 10 is an aberration curve diagram for a short-distance object point (object point distance of 100 mm) according to the first embodiment of the present invention.
FIG. 11 is an aberration curve diagram for an object point at infinity according to the second embodiment of the present invention.
FIG. 12 is an aberration curve diagram for a short-distance object point (object point distance 100 mm) according to the second embodiment of the present invention.
FIG. 13 is an aberration curve diagram for a short-distance object point (object point distance 11 mm) according to the third embodiment of the present invention.
FIG. 14 is an aberration curve diagram for an object point at infinity according to a fourth embodiment of the present invention.
FIG. 15 is an aberration curve diagram for an object point at infinity according to a fifth embodiment of the present invention.
FIG. 16 is an aberration curve diagram for an object point at infinity according to a sixth embodiment of the present invention.
FIG. 17 is an aberration curve diagram for a short-distance object point (object distance 2.85 mm) according to the seventh embodiment of the present invention.
FIG. 18 is an aberration curve diagram for an object point at infinity according to Example 8 of the present invention.
FIG. 19 is an aberration curve diagram with respect to a short-distance object point (object distance 100 mm) according to the eighth embodiment of the present invention.
FIG. 20 is a graph showing the relationship between the refractive power ratio a of a medium and the axial chromatic aberration and Petzval sum.
FIG. 21 is a diagram showing a distribution of refractive index-Abbe number of a glass material.
FIG. 22 is a diagram showing an example of a combination of a radial type gradient index lens and a homogeneous lens.
FIG. 23 is a diagram showing a means for adhering a flat surface of a radial type gradient index lens to a concave surface side of a homogeneous lens.
FIG. 24 is a diagram showing another means for closely attaching the flat surface of the radial type gradient index lens to the concave surface side of the homogeneous lens.
FIG. 25 is a diagram showing an example in which spherical processing is directly performed on a radial type gradient index lens which is a conventional manufacturing example.
FIG. 26 is a diagram showing a combination example of the imaging lens of the present invention.
FIG. 27 is a diagram showing an example of incorporating the imaging lens of the present invention into a lens frame.
FIG. 28 is a diagram showing another example of incorporating the imaging lens of the present invention into a lens barrel.

Claims (7)

A radial type refractive index distribution lens represented by the following formula (a) having a refractive index distributed in a radial direction from an optical axis having a positive refractive power and a homogeneous lens having a negative refractive power; The imaging lens, characterized in that the gradient index lens has a biplanar shape, and a part or all of the surface of the homogeneous lens on the radial type gradient index lens side has a planar shape, and satisfies the following condition (2). .
N (r) = N ₀₀ + N ₁₀ r ² + N ₂₀ r ⁴ (a)
(2) 1 / V ₁₀ <1 / ν _h
Here, r is the distance in the radial direction from the optical axis, N (r) is the refractive index distribution at the point of the distance r, _Ni0 is the 2i-order refractive index distribution coefficient, and _Vi0 is i of the radial type gradient index lens. It represents the following dispersion and is given by the following equations (b) and (c), where ν _h is the Abbe number of the homogeneous lens.
V ₀₀ = (N _00d −1) / (N _00F −N _00C ) (i = 0) (b)
_{V i0 = N i0d / (N} i0F -N i0C) (i = 1,2,3 ··) (c)
Here, N _00d , N _00F , and N _00C are the refractive indices on the optical axis of the d, F, and C lines, respectively, and _Ni0d , _Ni0F , and _Ni0C are the 2i-order refractive index distributions of the d, F, and C lines, respectively. It is a coefficient. A homogeneous lens having a negative refractive power and a radial type gradient index lens represented by the following formula (a) having a refractive index distributed in a radial direction from an optical axis having a positive refractive power; And focusing on an object at a short distance by changing the distance between the lens and the radial type gradient index lens.
N (r) = N ₀₀ + N ₁₀ r ² + N ₂₀ r ⁴ (a)
Here, r is the distance in the radial direction from the optical axis, N (r) is the refractive index distribution at the point of the distance r, and _Ni0 is the 2i-order refractive index distribution coefficient. 3. The imaging lens according to claim 1, wherein the condition (3) is satisfied.
(3) −1.5 <f / f _h <−0.05
Where, f is the focal length of the focal length, f _h is homogeneous lens of the taking lens system. The imaging lens according to claim 1, wherein the condition (4) is satisfied.
(4) 0.5 <t _G / f <9
Here, t _G is the thickness of the radial type gradient index lens, and f is the focal length of the entire imaging lens system. The imaging lens according to claim 1, wherein the condition (6) is satisfied.
(6) −0.01 <1 / V ₁₀ <0.008 The imaging lens according to claim 1, wherein the condition (9) is satisfied.
(9) 1 / ν _h <0.03
Here, ν _h is the Abbe number of the homogeneous lens. The imaging lens according to claim 1, wherein the condition (12) is satisfied.
(12) 0.5 <a _PAC <1.7

Images