JP4382370B2 - Variable magnification imaging lens and image reading apparatus using the same - Google Patents

Description

【００６７】
ここで、再び設計手法の説明に戻る。仮想レンズを実形状に変換するには、光路差関数をΔ、ｎ次の係数をＡｎ、光軸からの高さをｈとしたとき、次式（１２）を用いる。
【００６８】
【数１２】
Δ（ｈ）＝Ａ_２×ｈ^２＋Ａ_４×ｈ^４＋Ａ_６×ｈ^６＋… （１２）
【００６９】
この光路差関数式に従い、光路差が波長λの整数倍になる毎に基板面に輪帯を形成してゆく。
【００７０】
また、形成された鋸歯状輪帯の深さ方向（格子高さ）Ｌは、回折光学素子の基板材料の屈折率をｎ´としたとき、次式（１３）で表される（但し、ｉ＝１，２，３，４…）。
【００７１】
【数１３】
Ｌ（ｈ）＝Δ（ｈ）−｛ｉλ／（ｎ´−１）｝ （１３）
【００７２】
なお、輪帯の格子高さの最大値ＬｍａｘはＬｍａｘ＝λ／（ｎ´−１）を、第ｉ番目の輪帯半径ｈｉはＬ（ｈｉ）＝Δ（ｈｉ）−｛ｉλ／（ｎ´−１）｝＝０を解けばそれぞれ求めることができる。
【００７３】
図９〜１２は、第１〜第４実施例で用いた各回折光学面の断面形状を示す図である。これらの図で示す鋸歯状輪帯をした回折光学面は、キノフォームと呼ばれる。図９〜１２において、横軸は０が回折光学面の中心部であり、数値が大きくなる程、有効径外周方向に離れていくことを示している。また、縦軸は０を回折光学面の最も低い位置として、回折格子の高さを示している。なお、回折光学面の形状は、各図に示すような形状が望ましいが、製造上の都合によっては階段状のバイナリー形状でもよい。ただし、バイナリー形状である場合は、回折効率の観点から量子化数は８バイナリー以上であることが好ましい。
【００７４】
（第１実施例）
以下、本発明の第１実施例について、図１、図２を用いて説明する。図１は、本発明の第１実施例に係る可変倍率撮像レンズのレンズ構成及び低倍率（ｐｏｓ１）から高倍率（ｐｏｓ４）への変倍における各レンズ群の移動軌跡を示す図である。
【００７５】
図１の可変倍率撮像レンズにおいて、前レンズ群Ｇｆは、物体側から順に並んだ、両凸レンズＬ１、物体側に凸面を向けた正メニスカスレンズＬ２と物体側に凸面を向けた負メニスカスレンズＬ３との貼り合わせレンズ、回折光学面を有する回折光学素子ＤＯＥから構成されて、正の屈折力を有している。また、後レンズ群Ｇｒは、物体側から順に並んだ、物体側に凹面を向けた正メニスカスレンズＬ４、物体側に凹面を向けた負メニスカスレンズＬ５、両凸レンズＬ６と両凹レンズＬ７との貼り合わせレンズ、両凸レンズＬ８から構成されて、正の屈折力を有している。そして、低倍率側から高倍率側への変倍に伴い、前レンズ群Ｇｆと後レンズ群Ｇｒの頂点間隔を縮める方向に、前レンズ群Ｇｆ及び後レンズ群Ｇｒを物体側に光軸上移動させている。また、前レンズ群Ｇｆと後レンズ群Ｇｒとの間において、前レンズ群Ｇｆの近傍に開口絞りＳが配置され、この開口絞りＳは変倍時に前レンズ群Ｇｆとともに移動する。
【００７６】
本発明の第１実施例における各レンズの諸元を表２に示す。設計基準波長（ｄ線）において、表中の、βは倍率、ＦｎｏはＦナンバー、Ｂｆはバックフォーカス、ｄ０は物点（物体面）Ｏからレンズ系の第１面までの間隔、面番号は物体側から数えたレンズ面の順番（面番号１〜１８は図１における符号１〜１８に対応している）、ｒはレンズ面の曲率半径（非球面の場合には基準球面の曲率半径）、ｄは各レンズ面の間隔、νｄは各ガラスのアッベ数、ＤＯＥは回折光学素子を各々示している。Ｎｄはｄ線、Ｎｇはｇ線、ＮＣはＣ線、ＮＡ´はＡ´線に対する屈折率を各々示している。なお、表２において、回折光学面は上述した非球面式で記述されており、面番号の右に＊印を付している。
【００７７】
なお、表２において、面番号７及び８が回折光学素子ＤＯＥを、面番号９が開口絞りＳ、面番号１０が後レンズ群第１レンズＬ４の第１面の曲率半径Ｒｒ１、面番号１１が後レンズ群第１レンズＬ４の第２面の曲率半径Ｒｒ２を示す。また、物点Ｏからレンズ系の第１面までの間隔ｄ０、面番号９に示す面間隔（すなわち面番号９と面番号１０との面間隔）ｄ９、面番号１８に示す面間隔（すなわち面番号１８と像面Ｉとの面間隔）であるバックフォーカスＢｆは、それぞれ（使用）倍率βに応じて変化する。そこで、このように変倍に伴い変化する値は、表２中の合焦間隔表において使用倍率ｐｏｓ１〜ｐｏｓ４毎に示している。さらに、表２において、前述の条件式（１）〜（６）に対応する値、すなわち条件対応値も示している。
【００７８】
なお、諸元を示す表において、曲率半径ｒ、面間隔ｄその他の長さの単位は、一般に「ｍｍ」が使われるが、光学系は比例拡大又は比例縮小しても同等の光学性能が得られるので、これに限られるものではない。以上の表の説明は、他の実施例においても同様である。
【００７９】
【表２】

【００８０】
このように第１実施例では、上記条件式（１）〜（６）は全て満たされることがわかる。
【００８１】
図２（ａ），（ｂ）は、第１実施例の諸収差図である。すなわち、図２（ａ）は最も低倍率(ｐｏｓ１)における諸収差図であり、図２（ｂ）は最も高倍率（ｐｏｓ４）における諸収差図である。各収差図において、ＮＡは開口数を、Ｙは像高を、ｄはｄ線を、ｇはｇ線を、ＣはＣ線を、ＦはＦ線を、ＡはＡ´線をそれぞれ示している。なお、球面収差図において開口数ＮＡの最大値を、非点収差図及び歪曲収差図においては像高の最大値をそれぞれ示している。また、非点収差図では実線はサジタル像面を示し、破線はメリディオナル像面を示している。以上の収差図の説明は、他の実施例においても同様である。
【００８２】
各収差図から明らかなように、第１実施例では、赤外線領域まで含んだ諸収差がバランス良く補正され、優れた結像性能が確保されていることがわかる。
【００８３】
（第２実施例）
次に、本発明の第２実施例について、図３、図４を用いて説明する。図３は、本発明の第２実施例に係る可変倍率撮像レンズのレンズ構成及び低倍率（ｐｏｓ１）から高倍率（ｐｏｓ４）への変倍における各レンズ群の移動軌跡を示す図である。
【００８４】
図３の可変倍率撮像レンズにおいて、前レンズ群Ｇｆは、物体側から順に並んだ、両凸レンズＬ１、両凸レンズＬ２と両凹レンズＬ３との貼り合わせレンズから構成されて、正の屈折力を有している。また、後レンズ群Ｇｒは、物体側から順に並んだ、回折光学面を有する回折光学素子ＤＯＥ、物体側に凹面を向けた正メニスカスレンズＬ４、物体側に凹面を向けた負メニスカスレンズＬ５と物体側に凹面を向けた正メニスカスレンズＬ６との貼り合わせレンズ、物体側に凹面を向けた正メニスカスレンズＬ７、両凹レンズＬ８と両凸レンズＬ９との貼り合わせレンズから構成されて、正の屈折力を有している。そして、低倍率側から高倍率側への変倍に伴い、前レンズ群Ｇｆと後レンズ群Ｇｒの頂点間隔を縮める方向に、前レンズ群Ｇｆ及び後レンズ群Ｇｒを物体側に光軸上移動させている。また、前レンズ群Ｇｆと後レンズ群Ｇｒとの間において、後レンズ群Ｇｒの近傍に開口絞りＳが配置され、この開口絞りＳは変倍時に後レンズ群Ｇｒとともに移動する。
【００８５】
このように図３に示した本発明の第２実施例における各レンズの諸元を表３に示す。表３において、面番号１〜１９は、図３における符号１〜１９に対応している。なお、表３において、面番号６が開口絞りＳ、面番号８及び９が回折光学素子ＤＯＥ、面番号１０が後レンズ群第１レンズＬ４の第１面の曲率半径Ｒｒ１、面番号１１が後レンズ群第１レンズＬ４の第２面の曲率半径Ｒｒ２を示す。また、物点Ｏからレンズ系の第１面までの間隔ｄ０、面番号５に示す面間隔（すなわち面番号５と面番号６との面間隔）ｄ５、面番号１９に示す面間隔（すなわち面番号１９と像面Ｉとの面間隔）であるバックフォーカスＢｆは、それぞれ（使用）倍率βに応じて変化する。そこで、このように変倍に伴い変化する値は、表３中の合焦間隔表において使用倍率ｐｏｓ１〜ｐｏｓ４毎に示している。さらに、表３において、前述の条件式（１）〜（６）に対応する値、すなわち条件対応値も示している。
【００８６】
【表３】

【００８７】
このように第２実施例では、上記条件式（１）〜（６）は全て満たされることがわかる。
【００８８】
図４（ａ），（ｂ）は、第２実施例の諸収差図である。図４（ａ）は最も低倍率(ｐｏｓ１)における諸収差図であり、図４（ｂ）は最も高倍率（ｐｏｓ４）における諸収差図である。各収差図から明らかなように、第２実施例では、赤外線領域まで含んだ諸収差がバランス良く補正され、優れた結像性能が確保されていることがわかる。
【００８９】
（第３実施例）
続いて、本発明の第３実施例について図５、図６を用いて説明する。図５は、本発明の第３実施例に係る可変倍率撮像レンズのレンズ構成及び低倍率（ｐｏｓ１）から高倍率（ｐｏｓ４）への変倍における各レンズ群の移動軌跡を示す図である。
【００９０】
図５の可変倍率撮像レンズにおいて、前レンズ群Ｇｆは、物体側から順に並んだ、物体側に凸面を向けた正メニスカスレンズＬ１、物体側に凸面を向けた正メニスカスレンズＬ２と物体側に凸面を向けた負メニスカスレンズＬ３との貼り合わせレンズから構成されて、正の屈折力を有している。また、後レンズ群Ｇｒは、物体側から順に並んだ、回折光学面を有する回折光学素子ＤＯＥ、物体側に凹面を向けた正メニスカスレンズＬ４、物体側に凹面を向けた負メニスカスレンズＬ５と物体側に凹面を向けた正メニスカスレンズＬ６との貼り合わせレンズ、両凸レンズＬ７と両凹レンズＬ８と両凸レンズＬ９との３枚貼り合わせレンズから構成されて、正の屈折力を有している。そして、低倍率側から高倍率側への変倍に伴い、前レンズ群Ｇｆと後レンズ群Ｇｒの頂点間隔を縮める方向に、前レンズ群Ｇｆ及び後レンズ群Ｇｒを物体側に光軸上移動させている。また、前レンズ群Ｇｆと後レンズ群Ｇｒとの間において、後レンズ群Ｇｒの近傍に開口絞りＳが配置され、この開口絞りＳは変倍時に後レンズ群Ｇｒとともに移動する。
【００９１】
このように図５に示した本発明の第３実施例における各レンズの諸元を表４に示す。表４において、面番号１〜１８は、図５における符号１〜１８に対応している。なお、表４において、面番号６が開口絞りＳ、面番号８及び９が回折光学素子ＤＯＥ、面番号１０が後レンズ群第１レンズＬ４の第１面の曲率半径Ｒｒ１、面番号１１が後レンズ群第１レンズＬ４の第２面の曲率半径Ｒｒ２を示す。また、物点Ｏからレンズ系の第１面までの間隔ｄ０、面番号５に示す面間隔（すなわち面番号５と面番号６との面間隔）ｄ５、面番号１９に示す面間隔（すなわち面番号１９と像面Ｉとの面間隔）であるバックフォーカスＢｆは、それぞれ（使用）倍率βに応じて変化する。そこで、このように変倍に伴い変化する値は、表４中の合焦間隔表において使用倍率ｐｏｓ１〜ｐｏｓ４毎に示している。さらに、表４において、前述の条件式（１）〜（６）に対応する値、すなわち条件対応値も示している。
【００９２】
【表４】

【００９３】
このように第３実施例では、上記条件式（１）〜（６）は全て満たされることがわかる。
【００９４】
図６（ａ），（ｂ）は、第３実施例の諸収差図である。図６（ａ）は低倍率(ｐｏｓ１)における諸収差図であり、図６（ｂ）は高倍率（ｐｏｓ４）における諸収差図である。各収差図から明らかなように、第３実施例では、赤外線領域まで含んだ諸収差がバランス良く補正され、優れた結像性能が確保されていることがわかる。
【００９５】
（第４実施例）
次に、本発明の第４実施例について、図７、図８を用いて説明する。図７は、本発明の第４実施例に係る可変倍率撮像レンズのレンズ構成及び低倍率（ｐｏｓ１）から高倍率（ｐｏｓ４）への変倍における各レンズ群の移動軌跡を示す図である。
【００９６】
図７の可変倍率撮像レンズにおいて、前レンズ群Ｇｆは、物体側から順に並んだ、物体側に凸面を向けた正メニスカスレンズＬ１、両凸レンズＬ２と両凹レンズＬ３との貼り合わせレンズから構成されて、正の屈折力を有している。また、後レンズ群Ｇｒは、物体側から順に並んだ、回折光学面を有する回折光学素子ＤＯＥ、物体側に凹面を向けた正メニスカスレンズＬ４、物体側に凹面を向けた負メニスカスレンズＬ５と物体側に凹面を向けた正メニスカスレンズＬ６との貼り合わせレンズ、両凸レンズＬ７と両凹レンズＬ８と両凸レンズＬ９との３枚貼り合わせレンズから構成されて、正の屈折力を有している。そして、低倍率側から高倍率側への変倍に伴い、前レンズ群Ｇｆと後レンズ群Ｇｒの頂点間隔を縮める方向に、前レンズ群Ｇｆ及び後レンズ群Ｇｆを物体側に光軸上移動させている。また、前レンズ群Ｇｆと後レンズ群Ｇｒとの間において、後レンズ群Ｇｆの近傍に開口絞りＳが配置され、この開口絞りＳは変倍時に後レンズ群Ｇｒとともに移動する。
【００９７】
このように図７に示した本発明の第４実施例における各レンズの諸元を表５に示す。表５において、面番号１〜１８は、図７における符号１〜１８に対応している。なお、表５において、面番号６が開口絞りＳ、面番号８及び９が回折光学素子ＤＯＥ、面番号１０が後レンズ群第１レンズＬ４の第１面の曲率半径Ｒｒ１、面番号１１が後レンズ群第１レンズＬ４の第２面の曲率半径Ｒｒ２を示す。また、物点Ｏからレンズ系の第１面までの間隔ｄ０、面番号５に示す面間隔（すなわち面番号５と面番号６との面間隔）ｄ５、面番号１８に示す面間隔（すなわち面番号１８と像面Ｉとの面間隔）であるバックフォーカスＢｆは、それぞれ（使用）倍率βに応じて変化する。そこで、このように変倍に伴い変化する値は、表５中の合焦間隔表において使用倍率ｐｏｓ１〜ｐｏｓ４毎に示している。さらに、表５において、前述の条件式（１）〜（６）に対応する値、すなわち条件対応値も示している。
【００９８】
【表５】

【００９９】
このように第４実施例では、上記条件式（１）〜（６）は全て満たされることがわかる。
【０１００】
図８（ａ），（ｂ）は、第４実施例の諸収差図である。図８（ａ）は最も低倍率(ｐｏｓ１)における諸収差図であり、図８（ｂ）は最も高倍率（ｐｏｓ４）における諸収差図である。各収差図から明らかなように、第４実施例では、赤外線領域まで含んだ諸収差がバランス良く補正され、優れた結像性能が確保されていることがわかる。
【０１０１】
本発明に係る可変倍率撮像レンズを透過原稿読取装置に適用した場合について説明する。透過原稿読取装置は、図１３に示すように、照明光源▲１▼と、原稿台▲２▼と、光路折り曲げ用ミラー▲３▼と、本発明に係る可変倍率撮像レンズ▲４▼と、読取用の固体撮像素子（ＣＣＤ等）▲５▼とから構成されている。このような構成の透過原稿読取装置は、原稿台▲２▼の面上に原稿（図示略）をセットすると、この原稿の大きさに応じて可変倍率撮像レンズ▲４▼の倍率を合わせるためにその位置を移動させ、固体撮像素子▲５▼も移動させ、適正な共役長にて結像させている。また、照明光源▲１▼から放射された光線は原稿台▲２▼の面上にセットされた原稿を照明し、この原稿の透過光線を光路折り曲げ用ミラー▲３▼にて折り曲げて可変倍率撮像レンズ▲４▼を通して、固体撮像素子▲５▼にて結像させている。そして、このような読取過程に伴って、原稿台▲２▼は、図中矢印ａの方向に移動して原稿全面の画像情報を読取っている。
【０１０２】
【発明の効果】
以上説明したように、本発明によれば、赤外線領域まで色収差を含む諸収差がバランス良く補正された撮影倍率を変倍比３以上で変化させることが可能な光学系を得ることができる。
【０１０３】
具体的には、本発明において、カラーコピー機、カラーファクシミリ、カラーイメージスキャナ、カラーフィルムスキャナ等に適した可変倍率撮像レンズ及びこれを含む画像読取装置を実現することができる。
【図面の簡単な説明】
【図１】本発明の第１実施例に係る可変倍率撮像レンズのレンズ構成を示す図である。
【図２】図２（ａ）は第１実施例の（最も）低倍率（ｐｏｓ１）における諸収差図であり、図２（ｂ）は第１実施例の（最も）高倍率（ｐｏｓ４）における諸収差図である。
【図３】本発明の第２実施例に係る可変倍率撮像レンズのレンズ構成を示す図である。
【図４】図４（ａ）は第２実施例の（最も）低倍率（ｐｏｓ１）における諸収差図であり、図４（ｂ）は第２実施例の（最も）高倍率（ｐｏｓ４）における諸収差図である。
【図５】本発明の第３実施例に係る可変倍率撮像レンズのレンズ構成を示す図である。
【図６】図６（ａ）は第３実施例の（最も）低倍率（ｐｏｓ１）における諸収差図であり、図６（ｂ）は第３実施例の（最も）高倍率（ｐｏｓ４）における諸収差図である。
【図７】本発明の第４実施例に係る可変倍率撮像レンズのレンズ構成を示す図である。
【図８】図８（ａ）は第４実施例の（最も）低倍率（ｐｏｓ１）における諸収差図であり、図８（ｂ）は第４実施例の（最も）高倍率（ｐｏｓ４）における諸収差図である。
【図９】第１実施例の回折光学面の鋸歯形状を示す図である。
【図１０】第２実施例の回折光学面の鋸歯形状を示す図である。
【図１１】第３実施例の回折光学面の鋸歯形状を示す図である。
【図１２】第４実施例の回折光学面の鋸歯形状を示す図である。
【図１３】本発明に係る可変倍率撮像レンズを透過原稿読取装置に適用したときの要部概略図である。
【符号の説明】
Ｇｆ 前レンズ群
Ｇｒ 後レンズ群
Ｒｒ１ 後レンズ群第１レンズの第１面の曲率半径
Ｒｒ２ 後レンズ群第１レンズの第２面の曲率半径
Ｓ 開口絞り
ＤＯＥ 回折光学素子
Ｏ 物点（物体面）
Ｉ 像面
▲１▼ 照明光源
▲２▼ 原稿台
▲３▼ 光路折り曲げ用ミラー
▲４▼ 可変倍率撮像レンズ
▲５▼ 固体撮像素子[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a variable-magnification imaging lens suitable for an optical system having a finite distance between an object plane and an image plane, such as a facsimile, an image scanner, a film scanner, and other copying machines, and capable of reading originals of various sizes, and The present invention relates to an image reading apparatus including the same.
[0002]
[Prior art]
In an image reading apparatus such as a facsimile or an image scanner, an image of a document is formed on a solid-state image pickup device such as a CCD at the same magnification or enlarged and reduced via a reading lens. In recent years, the use magnification of the reading lens has been increased due to the increase in the size of the solid-state imaging device accompanying the increase in resolution. Further, such a reading lens needs to have a good imaging performance in which curvature of field, astigmatism, and distortion are sufficiently corrected, and a light receiving element such as a CCD sensor to be used. It is necessary to have an equivalent resolution. In order to secure a sufficient amount of peripheral light and shorten the reading time, the reading lens needs to be bright.
[0003]
Further, an image sensor called a 3-line 1-chip CCD is used in recent color image reading apparatuses. In this sensor, light receiving elements having three color filters of red (R), green (G), and blue (B) are arranged on one chip, and an image of a color original is formed on the light receiving surface of the light receiving element. By imaging, the color image information of the three colors is converted into a signal at a time. However, when the three-color image information is read at a time in this way, the imaging positions along the optical axes of the respective colors must be matched. This is because if the image formation position along the optical axis where the resolution is highest is greatly shifted for each color, the resolution of blue (B) is low even if the resolution of red (R) is high at a certain image formation position. This is because a problem occurs. Therefore, in order to obtain a high MTF value for each color at any imaging position, chromatic aberration (particularly axial chromatic aberration) must be corrected over a wide wavelength range with a good balance, and the secondary spectrum must be small. Furthermore, in order to capture an image in the infrared region (including the near infrared region) for compensating for the influence (dust, scratches, etc.) of the recording medium, a chromatic aberration balance in which the light in the infrared region (R ′) is also corrected is required. ing.
[0004]
The size of a document or film read by a reading optical device such as a scanner is not constant, and there are various sizes. For this reason, the reading lens preferably has a variable shooting magnification. Here, a reading lens such as a scanner may not necessarily maintain a constant distance between object images when changing the photographing magnification. For example, even if a document or a film is fixed, the distance between the object images does not have to be always constant if the solid-state imaging device moves together with the reading lens optical system so as to maintain the imaging relationship at the time of zooming. In order to deal with documents of various sizes, it is necessary that fluctuations in various aberrations accompanying changes in photographing magnification are small.
[0005]
Conventionally, there are reading optical devices that use lenses with a constant distance between object images, so-called zoom lenses, and reading optical systems limited to originals of a specific size, that is, reading optical devices that use a plurality of optical systems with a fixed reading magnification. Are known. However, since a reading optical device such as a film scanner requires high resolution, the zoom lens used here has a large number of lenses, resulting in an increase in the size of the device itself. Similarly, devices using a plurality of optical systems are similarly increased in size and cost. In order to avoid such an increase in the size of the device, there are cases where it must be used only at a limited magnification.
[0006]
In addition, as a macro lens for photography, many lenses that can be used from an object at infinity to the same magnification photographing are conventionally known. However, since the macro lens is a lens designed for taking a picture on a film, when the macro lens is used for a reading device, there are problems such as insufficient resolution and insufficient amount of peripheral light.
[0007]
Conventionally, various lens systems have been proposed in response to the above requirements. For example, in order to solve the problem of chromatic aberration, a lens system that suppresses the secondary spectrum satisfactorily (see, for example, Patent Documents 1 and 2), a diffractive optical surface or an element having a diffractive optical surface (hereinafter referred to as a diffractive optical element). (For example, see Patent Document 3). In addition, a lens system capable of zooming that satisfactorily solves problems such as resolving power and insufficient amount of peripheral light is disclosed (for example, see Patent Documents 4 to 7).
[0008]
[Patent Document 1]
Japanese Patent Laid-Open No. 10-206731
[Patent Document 2]
JP 2000-89108 A
[Patent Document 3]
Japanese Patent Laid-Open No. 10-339843
[Patent Document 4]
JP-A-5-11189
[Patent Document 5]
JP-A-5-273465
[Patent Document 6]
JP-A-6-94993
[Patent Document 7]
JP 2000-275516 A
[0009]
[Problems to be solved by the invention]
However, in the above-mentioned known example, the variable magnification for reading has a zoom ratio of 3 times or more, the fluctuation of various aberrations accompanying the zoom is small, and the chromatic aberration up to the infrared region is corrected very well. An imaging lens was not disclosed.
[0010]
In the lens system described in Patent Document 1, chromatic aberration is corrected well, but the use magnification is fixed. Further, in the lens system described in Patent Document 2, chromatic aberration is corrected very well up to the infrared region, but the use magnification is fixed. Similarly, in the lens system described in Patent Document 3, chromatic aberration is favorably corrected by using a diffractive optical surface, but the use magnification is fixed.
[0011]
The lens system described in Patent Document 4 is a lens system for a copying machine such as a copying machine, and does not have the high resolution aimed by the present invention.
[0012]
In the lens system described in Patent Document 6, the magnification used and the zoom ratio (the range of usable imaging magnification) were small.
[0013]
The lens system described in Patent Document 5 has a zoom ratio close to 3 ×, but the fluctuations of various aberrations accompanying this zoom are large.
[0014]
The lens system described in Patent Document 7 has a zoom ratio exceeding 3 times, but the chromatic aberration correction remains in the visible light region, and the correction amount of chromatic aberration is hardly sufficient.
[0015]
The present invention has been made in view of such problems, and in the process of zooming from the low magnification side to the high magnification side, there are few variations in various aberrations, the zoom ratio of the imaging magnification is large, and axial chromatic aberration. An object of the present invention is to provide a bright variable-magnification imaging lens capable of reading a color original and the like and an image reading apparatus including the lens.
[0016]
[Means for Solving the Problems]
  In order to solve the above problem, a variable magnification imaging lens according to the present invention includes a front lens group having positive refractive power, arranged in order from the object side, and positive refractive power.TheA rear lens group, and a diffractive optical element having at least one diffractive optical surface;Consist of,
  As the magnification is changed from the low magnification side to the high magnification side, both the front lens group and the rear lens group are moved to the object side while changing the air gap between the front lens group and the rear lens group.,
The diffractive optical element is configured to move together with the front lens group or the rear lens group upon zooming.,
  DF represents the amount of movement due to zooming from the lowest magnification to the maximum magnification of the front lens group, DR represents the amount of movement due to zooming from the lowest magnification to the maximum magnification of the rear lens group, and the focal length of the diffractive optical surface is FDOE, When the combined focal length at the minimum magnification of the entire lens system is Fa, the following conditional expression
              0.7 <DF / DR <1.0
              0.03 <Fa / FDOE <0.1
Satisfied,
  The diffractive optical surface has the following conditional expression where hp is the height of the principal ray with respect to the maximum image height incident on the diffractive optical surface, and y is the maximum image height y in all the zooming regions.
              | Hp / y | <0.1
It is arranged to satisfy.
[0018]
In the variable magnification imaging lens according to the present invention, the first lens in the rear lens group located closest to the object side in the rear lens group preferably has a meniscus lens shape with a concave surface facing the object side.
[0021]
In the variable magnification imaging lens according to the present invention, an aperture stop is disposed between the front lens group and the rear lens group so that the distance between the diffractive optical surface and the aperture stop is always kept constant during zooming. It is desirable to be configured.
[0022]
An image reading apparatus according to the present invention is configured to include any of the variable magnification imaging lenses described above.
[0023]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of a variable magnification imaging lens and an image reading apparatus including the same according to the present invention will be described. In the present invention, a front lens group having a positive refractive power, a rear lens group having a positive refractive power, and a diffractive optical element having at least one diffractive optical surface, which are arranged in order from the object side, Variable magnification imaging so that both the front lens group and the rear lens group are moved to the object side while changing the air gap between the front lens group and the rear lens group as the magnification is changed from the low magnification side to the high magnification side. The lens is configured.
[0024]
In the present invention having such a configuration, in order to secure a wide range of use magnification (because the zoom ratio of the variable magnification imaging lens is large), the zoom lens is changed from the object to the front lens group as the zoom ratio is changed from the low magnification side to the high magnification. Spherical aberration caused by zooming by adopting a so-called floating method (distance correction) in which the distance between the front lens group and the rear lens group is changed and moved to the object side while reducing the distance to one surface. Variations such as coma and astigmatism are corrected. However, with the above floating method alone, axial chromatic aberration increases and becomes difficult to suppress as the use magnification increases. Therefore, conventionally, special low dispersion glass is used to suppress the longitudinal chromatic aberration.
[0025]
However, special low-dispersion glass has a low refractive power, so when trying to obtain a certain refractive power, the radius of curvature is smaller (tighter) than glass with a high refractive index, and correction of spherical aberration bending, etc. It has become difficult to make the lens system brighter.
[0026]
In addition, in the special low dispersion glass, the glass that can be used for correcting the longitudinal chromatic aberration is limited, so that the degree of freedom of Petzval sum is lost and it is difficult to correct curvature of field. As a result, when trying to increase the zoom ratio of the variable magnification, correction for variations in various aberrations has been insufficient.
[0027]
Therefore, in the present invention, axial chromatic aberration is corrected by using a diffractive optical element having a diffractive optical surface. As a result, the degree of freedom of selection is increased in the glass material used for the lens constituting the present invention without using a special low dispersion glass having a low refractive index. Furthermore, the Petzval sum can be corrected by increasing the degree of freedom in selecting the glass material, and the variation of various aberrations due to field curvature correction and zooming is suppressed, and a bright variable magnification imaging lens with a high zoom ratio is realized. Yes.
[0028]
In the present invention, the rear lens group moves in the direction of reducing the distance between the front lens group and the rear lens group while reducing the distance from the object to the first surface of the front lens group. Floating method (interval correction) is used to zoom in. When the axial air space (vertex space) between the front lens group and the rear lens group having positive refractive power is reduced, the combined focal length of the entire variable magnification imaging lens system is shortened. As a result, the execution F number on the high magnification side is reduced from increasing (that is, darkening).
[0029]
In general, the secondary spectrum in axial chromatic aberration increases in proportion to the square of the lateral magnification, and increases as the combined focal length increases. Therefore, in the present invention, by moving to the object side so as to reduce the distance between the front lens group and the rear lens group, the combined focal length is shortened, and the increase in the secondary spectrum is mitigated. Furthermore, by reducing the distance between the front lens group and the rear lens group, it is possible to secure a sufficient amount of peripheral light in non-vignetting.
[0030]
In the rear lens group of the variable magnification imaging lens according to the present invention, the first lens in the rear lens group located closest to the object side preferably has a meniscus lens shape with a concave surface facing the object side. By adopting such a shape, when the light beam that has passed through the front lens group enters the rear lens group, an angle close to perpendicular to the surface of the first lens in the rear lens group can be taken. As a result, it is possible to avoid excessive bending of the light beam, and therefore the present invention realizes that various aberrations are corrected well to suppress fluctuation due to zooming.
[0031]
Hereinafter, the variable magnification imaging lens of the present invention will be described in detail along the description of the conditional expression. In the variable magnification imaging lens of the present invention, the amount of movement by zooming from the lowest magnification of the front lens group to the maximum magnification is DF, and the amount of movement of the rear lens group by zooming from the lowest magnification to the maximum magnification is DR. The following conditional expression (1) is satisfied.
[0032]
[Expression 1]
0.7 <DF / DR <1.0 (1)
[0033]
The above conditional expression (1) is a conditional expression for correcting various aberrations in a well-balanced manner in all the variable magnification regions from the low magnification side to the high magnification side. The front lens is obtained by changing the magnification from the lowest magnification to the highest magnification. The optimum ratio of the amount of movement of the lens group and the rear lens group is defined. If the upper limit of conditional expression (1) is exceeded, the amount of movement of the front lens group with respect to the rear lens group increases, the secondary spectrum at high magnification increases, and the brightness on the high magnification side is ensured as described above. Is not preferable because it becomes difficult. On the other hand, if the lower limit value of conditional expression (1) is not reached, the amount of movement of the rear lens unit with respect to the front lens unit due to zooming is too large, and variations in various aberrations such as spherical aberration, coma and astigmatism are large. Become. Further, since the total length of the lens at the minimum magnification becomes long, it is necessary to increase the effective diameter in order to secure a sufficient amount of peripheral light. As the effective diameter increases, the mechanism for moving the lens also increases, which is not preferable because it is contrary to the size reduction aimed at by the present invention.
[0034]
Furthermore, it is desirable that the movement ratio between the front lens group and the rear lens group in the floating condition in the conditional expression (1) does not change with zooming (always constant). If the ratio of the amount of movement of the rear lens group to the front lens group changes during zooming from the low magnification side to the high magnification side, a complex structure such as a non-linear cam is used as a mirror to realize the movement of the lens group. This is because it is required for the cylinder and is not preferable. However, it is also possible to optimize aberration fluctuation correction by changing the movement ratio between the front lens group and the rear lens group in accordance with zooming.
[0035]
  In the variable magnification imaging lens according to the present invention, the diffractive optical element moves together with the front lens group or the rear lens group at the time of zooming. When the focal length is Fa, the following conditional expression (2) must be satisfiedis necessary.
[0036]
[Expression 2]
0.03 <Fa / FDOE <0.1 (2)
[0037]
The conditional expression (2) is an expression indicating an appropriate range of refractive power of the diffractive optical surface used in the present invention. If the upper limit of conditional expression (2) is exceeded, the influence of the diffractive optical surface on the entire imaging lens system will be large, and chromatic aberration correction will be excessive, and other aberrations (not chromatic aberration) will be affected. At the same time, it becomes difficult to form a diffractive optical surface having a simple structure. On the other hand, if the lower limit of conditional expression (2) is not reached, it is almost equivalent to chromatic aberration correction performed using only the refractive optical system, and correction of chromatic aberration that covers all the variable power regions in the lens of the present invention is insufficient. . The refractive power of the diffractive optical surface is preferably positive.
[0038]
  In the variable-magnification imaging lens according to the present invention, the diffractive optical surface is disposed at all the variable magnification regions at the principal ray height with respect to the maximum image height incident on the diffractive optical surface, hp, and the maximum image height y. When satisfying the following conditional expression (3)is necessary.
[0039]
[Equation 3]
| Hp / y | <0.1 (3)
[0040]
In order to ensure good performance over the entire screen, in addition to correcting axial chromatic aberration, it is indispensable to properly correct lateral chromatic aberration around the screen. For example, assume that a diffractive optical element having one diffractive optical surface is combined with a plurality of refractive optical elements. The diffractive optical element and the refractive optical element have greatly different dispersion characteristics with respect to the wavelength. On the other hand, lateral chromatic aberration is greatly affected by the height of the principal ray from the optical axis. This is hardly a problem in an optical system in which the use magnification is fixed. However, in an optical system in which the use magnification is variable (a wide variable magnification region) as in the present invention, when a diffractive optical element having a diffractive optical surface is arranged, the incident principal ray height hp relative to the maximum image height is If the lens is disposed at a position that greatly changes due to zooming, the lateral chromatic aberration greatly varies accordingly. It is difficult to correct this, and this is not preferable because it causes an increase in the number of lenses and complicated floating.
[0041]
The conditional expression (3) is a conditional expression that represents the relationship between the incident principal ray height and the maximum image height with respect to the maximum image height at the position of the diffractive optical surface. An appropriate position is specified. If the range of the conditional expression (3) is exceeded, the influence of the diffractive optical surface on the lateral chromatic aberration is strong, resulting in overcorrection or undercorrection. Therefore, it is preferable that the conditional expression (3) is satisfied over the entire zooming region of the variable magnification imaging lens. That is, the lateral chromatic aberration can be corrected by the refractive optical system, and the axial chromatic aberration can be corrected by the diffractive optical surface.
[0042]
Furthermore, in the variable magnification imaging lens according to the present invention, it is more desirable that the place where the diffractive optical surface is disposed satisfies the following conditional expression (4).
[0043]
[Expression 4]
| Hp / y | <0.04 (4)
[0044]
If the range of the conditional expression (4) is satisfied, the influence on the spherical aberration and the like is further reduced, and an optimum diffractive optical surface can be obtained.
[0045]
In the present invention, an aperture stop is disposed between the front lens group having a positive refractive power and the rear lens group having a positive refractive power, and the diffractive optical surface and the aperture stop are being zoomed. It is preferable that the distance is always constant. With such an arrangement, the entire lens system becomes an arrangement close to a symmetric system, and it is possible to reduce fluctuations in lateral chromatic aberration and distortion as is well known. Further, by changing the distance of the element having the diffractive optical surface at the time of zooming, it becomes possible to reduce fluctuations in various aberrations.
[0046]
At this time, the diffractive optical element having the diffractive optical surface may be arranged on either the object side or the image side of the aperture stop. Furthermore, the diffractive optical surface of the diffractive optical element may be provided on either the object side or the image side. Further, even if a diffractive optical surface is provided on the lens surface of the refractive lens, the same effect can be obtained as an independent diffractive optical element.
[0047]
Furthermore, in the variable magnification imaging lens of the present invention, it is more desirable to satisfy the following conditional expressions (5) and (6).
[0048]
In the present invention, the axial air separation distance between the front lens group and the rear lens group at the maximum magnification is Dh, and the distance on the optical axis from the first surface to the final surface at the maximum magnification of the variable magnification imaging lens is expressed as (lens length). ) ΣDh, it is preferable that the following conditional expression (5) is satisfied.
[0049]
[Equation 5]
(ΣDh + Dh) / ΣDh <1.1 (5)
[0050]
The conditional expression (5) is defined in order to achieve miniaturization. Exceeding the upper limit value of conditional expression (5) indicates that the distance between the front lens group and the rear lens group with respect to the lens length at the maximum magnification becomes wide, and optimization for achieving miniaturization is not performed. Therefore, it is not preferable.
[0051]
In the present invention, when the radius of curvature of the first surface of the first lens in the rear lens group is Rr1, and the radius of curvature of the second surface of the first lens in the rear lens group is Rr2, the following conditional expression (6) is satisfied. Is preferred.
[0052]
[Formula 6]
1.0 <Rr1 / Rr2 <1.5 (6)
[0053]
Conditional expression (6) is an expression showing the optimum shape of the first lens in the rear lens group located closest to the front lens group in the rear lens group. If the upper limit value of conditional expression (6) is exceeded, the degree of convergence of rays incident on the rear lens group will increase, and variations in various aberrations (particularly coma and astigmatism) caused by zooming will increase. If the lower limit of conditional expression (6) is not reached, various aberrations increase, which is not preferable.
[0054]
【Example】
Embodiments of a variable magnification imaging lens according to the present invention will be described with reference to the accompanying drawings. Here, in an element having a diffractive optical surface used in each example, assuming that the refractive power of the first-order diffracted light (m = 1) is φ, the phase difference φ of the diffractive optical surface is expressed by the following equation (7). (Where i = 1, 2, 3, 4,...).
[0055]
[Expression 7]
φ (h) = (2π · m / λ) · (ΣA (2 · i) · h^(2.i)(7)
[0056]
In addition, said Formula (7) is represented as (phi) =-2 * A2 when the coefficient of a quadratic term is set to A2. Further, when the arbitrary wavelength is λ ′ and the reference wavelength is λ0, the change in refractive power of the arbitrary wavelength with respect to the reference wavelength is expressed by the following equation (8).
[0057]
[Equation 8]
Δφ = (λ ′ / λ0) · (−2 · A2) (8)
[0058]
Thus, it can be understood that a large dispersion can be obtained with a weak change in the paraxial refractive power by changing A2 as a characteristic of the diffractive optical surface. This means that it is possible to correct chromatic aberration without significantly affecting various aberrations other than chromatic aberration.
[0059]
A design method of the diffractive optical element will be described. For example, as disclosed in “Introduction to Diffraction Optics” (Applied Physics Society, Japan Optical Society, Optical Design Research Group; Optronics), the diffractive optical element design method includes a lattice model method and a high refractive index model method. There is. Both methods treat the diffractive optical surface as a virtual phase conversion surface having no actual shape at the design stage, and take the procedure of replacing the phase function with the actual shape at the final stage of design. In this embodiment, a method (high refractive index model method) for designing a diffractive optical element using a virtual refractive lens having an extremely high refractive index is used. Assuming that the design reference wavelength (d-line) λ0 = 587.56 nm and the virtual refractive index n = 10001, the refractive index n (λ ′) at an arbitrary wavelength λ ′ is expressed by the following equation (9).
[0060]
[Equation 9]
n (λ ′) = 1+ (λ ′ / λ0). (n−1) (9)
[0061]
A diffractive optical surface by the high refractive index method is expressed using a normal aspherical expression. In this equation, the height in the direction perpendicular to the optical axis is h, the amount of displacement in the optical axis direction at height h is S (h), the radius of curvature of the reference spherical surface is r, the conic coefficient is K, and the nth-order aspherical surface. When the coefficient is Cn, it is defined by the following equation (10).
[0062]
[Expression 10]
S (h) = (h²/ R) / [1+ {1-K · (h / r)²}^1/2]
+ C₂・ H²+ C₄・ H⁴+ C₆・ H⁶+ ... (10)
[0063]
The paraxial radius of curvature R of the aspherical surface is defined by the following equation (11).
[0064]
## EQU11 ##
R = 1 / {(1 / r) + 2C₂} (11)
[0065]
In each embodiment, the d-line, g-line, C-line, and A′-line are selected as the aberration characteristic calculation targets. Table 1 below shows the wavelengths of these d-line, g-line, C-line, and A′-line, and specific refractive index values set for each spectral line based on the high refractive index method described above.
[0066]
[Table 1]

[0067]
Here, it returns to description of a design method again. In order to convert the virtual lens into a real shape, when the optical path difference function is Δ, the n-th order coefficient is An, and the height from the optical axis is h, the following equation (12) is used.
[0068]
[Expression 12]
Δ (h) = A₂× h²+ A₄× h⁴+ A₆× h⁶+ ... (12)
[0069]
In accordance with this optical path difference function equation, an annular zone is formed on the substrate surface every time the optical path difference becomes an integral multiple of the wavelength λ.
[0070]
Further, the depth direction (lattice height) L of the formed serrated annular zone is expressed by the following formula (13) where n ′ is the refractive index of the substrate material of the diffractive optical element (provided that i = 1, 2, 3, 4 ...).
[0071]
[Formula 13]
L (h) = Δ (h) − {iλ / (n′−1)} (13)
[0072]
The maximum value Lmax of the lattice height of the annular zone is Lmax = λ / (n′−1), and the i-th annular zone radius hi is L (hi) = Δ (hi) − {iλ / (n ′ -1)} = 0 can be obtained respectively.
[0073]
9 to 12 are diagrams showing cross-sectional shapes of the diffractive optical surfaces used in the first to fourth examples. The diffractive optical surface having a serrated ring zone shown in these drawings is called a kinoform. 9 to 12, the horizontal axis indicates that 0 is the central portion of the diffractive optical surface, and the larger the numerical value, the farther away from the outer peripheral direction of the effective diameter. The vertical axis indicates the height of the diffraction grating with 0 being the lowest position on the diffractive optical surface. The shape of the diffractive optical surface is preferably as shown in each drawing, but may be a stepped binary shape depending on the manufacturing convenience. However, in the case of a binary shape, the quantization number is preferably 8 binary or more from the viewpoint of diffraction efficiency.
[0074]
(First embodiment)
A first embodiment of the present invention will be described below with reference to FIGS. FIG. 1 is a diagram illustrating a lens configuration of a variable magnification imaging lens according to a first example of the present invention and a movement locus of each lens group in zooming from low magnification (pos1) to high magnification (pos4).
[0075]
In the variable magnification imaging lens of FIG. 1, the front lens group Gf includes a biconvex lens L1, a positive meniscus lens L2 having a convex surface facing the object side, and a negative meniscus lens L3 having a convex surface facing the object side. And a diffractive optical element DOE having a diffractive optical surface, and has a positive refractive power. The rear lens group Gr is a combination of a positive meniscus lens L4 having a concave surface facing the object side, a negative meniscus lens L5 having a concave surface facing the object side, and a biconvex lens L6 and a biconcave lens L7 arranged in order from the object side. It is composed of a lens and a biconvex lens L8 and has a positive refractive power. As the magnification is changed from the low magnification side to the high magnification side, the front lens group Gf and the rear lens group Gr are moved on the optical axis to the object side in a direction to reduce the vertex interval between the front lens group Gf and the rear lens group Gr. I am letting. In addition, an aperture stop S is disposed near the front lens group Gf between the front lens group Gf and the rear lens group Gr, and the aperture stop S moves together with the front lens group Gf at the time of zooming.
[0076]
Table 2 shows the specifications of each lens in the first example of the present invention. In the design reference wavelength (d-line), β in the table is the magnification, Fno is the F-number, Bf is the back focus, d0 is the distance from the object point (object surface) O to the first surface of the lens system, and the surface number is The order of the lens surfaces counted from the object side (surface numbers 1 to 18 correspond to reference numerals 1 to 18 in FIG. 1), r is the radius of curvature of the lens surface (in the case of an aspherical surface, the radius of curvature of the reference spherical surface) , D is the distance between the lens surfaces, νd is the Abbe number of each glass, and DOE is the diffractive optical element. Nd represents the d-line, Ng represents the g-line, NC represents the C-line, and NA ′ represents the refractive index with respect to the A′-line. In Table 2, the diffractive optical surface is described by the aspherical formula described above, and an asterisk (*) is added to the right of the surface number.
[0077]
In Table 2, the surface numbers 7 and 8 are the diffractive optical element DOE, the surface number 9 is the aperture stop S, the surface number 10 is the radius of curvature Rr1 of the first surface of the rear lens group first lens L4, and the surface number 11 is The curvature radius Rr2 of the 2nd surface of the rear lens group 1st lens L4 is shown. Further, the distance d0 from the object point O to the first surface of the lens system, the surface distance indicated by the surface number 9 (namely, the surface distance between the surface number 9 and the surface number 10) d9, and the surface distance indicated by the surface number 18 (namely, the surface). The back focus Bf, which is the surface interval between the number 18 and the image plane I, changes in accordance with the (use) magnification β. Thus, the values that change with zooming are shown for each of the use magnifications pos1 to pos4 in the focusing interval table in Table 2. Further, Table 2 also shows values corresponding to the above-described conditional expressions (1) to (6), that is, condition corresponding values.
[0078]
In the table showing the specifications, the unit of length of curvature radius r, surface spacing d and other lengths is generally “mm”, but the optical system can obtain the same optical performance even if proportionally enlarged or reduced. It is not limited to this. The description of the above table is the same in other embodiments.
[0079]
[Table 2]

[0080]
Thus, in the first embodiment, it can be seen that all the conditional expressions (1) to (6) are satisfied.
[0081]
2A and 2B are graphs showing various aberrations of the first example. 2A is an aberration diagram at the lowest magnification (pos1), and FIG. 2B is an aberration diagram at the highest magnification (pos4). In each aberration diagram, NA is the numerical aperture, Y is the image height, d is the d line, g is the g line, C is the C line, F is the F line, and A is the A ′ line. Yes. In the spherical aberration diagram, the maximum value of the numerical aperture NA is shown, and in the astigmatism diagram and the distortion diagram, the maximum value of the image height is shown. In the astigmatism diagram, the solid line indicates the sagittal image plane, and the broken line indicates the meridional image plane. The explanation of the above aberration diagrams is the same in the other examples.
[0082]
As is apparent from each aberration diagram, in the first example, it is understood that various aberrations including the infrared region are corrected in a well-balanced manner and excellent imaging performance is ensured.
[0083]
(Second embodiment)
Next, a second embodiment of the present invention will be described with reference to FIGS. FIG. 3 is a diagram showing the lens configuration of the variable magnification imaging lens according to the second embodiment of the present invention and the movement trajectory of each lens group in zooming from low magnification (pos1) to high magnification (pos4).
[0084]
In the variable magnification imaging lens of FIG. 3, the front lens group Gf is composed of a biconvex lens L1, a cemented lens of a biconvex lens L2, and a biconcave lens L3 arranged in order from the object side, and has a positive refractive power. ing. The rear lens group Gr includes, in order from the object side, a diffractive optical element DOE having a diffractive optical surface, a positive meniscus lens L4 having a concave surface facing the object side, a negative meniscus lens L5 having a concave surface facing the object side, and an object The lens is composed of a cemented lens with a positive meniscus lens L6 having a concave surface facing the side, a positive meniscus lens L7 having a concave surface facing the object side, and a cemented lens with a biconcave lens L8 and a biconvex lens L9. Have. As the magnification is changed from the low magnification side to the high magnification side, the front lens group Gf and the rear lens group Gr are moved on the optical axis to the object side in a direction to reduce the vertex interval between the front lens group Gf and the rear lens group Gr. I am letting. An aperture stop S is disposed between the front lens group Gf and the rear lens group Gr and in the vicinity of the rear lens group Gr. The aperture stop S moves together with the rear lens group Gr during zooming.
[0085]
Table 3 shows the specifications of the lenses in the second embodiment of the present invention shown in FIG. In Table 3, surface numbers 1 to 19 correspond to reference numerals 1 to 19 in FIG. In Table 3, the surface number 6 is the aperture stop S, the surface numbers 8 and 9 are the diffractive optical element DOE, the surface number 10 is the radius of curvature Rr1 of the first surface of the rear lens group first lens L4, and the surface number 11 is the rear. The curvature radius Rr2 of the 2nd surface of the lens group 1st lens L4 is shown. Further, the distance d0 from the object point O to the first surface of the lens system, the surface distance indicated by the surface number 5 (namely, the surface distance between the surface number 5 and the surface number 6) d5, the surface distance indicated by the surface number 19 (namely, the surface). The back focus Bf, which is the surface interval between the number 19 and the image plane I, changes in accordance with the (use) magnification β. Therefore, the values that change with zooming in this way are shown for each use magnification pos1 to pos4 in the focusing interval table in Table 3. Further, in Table 3, values corresponding to the conditional expressions (1) to (6) described above, that is, condition corresponding values are also shown.
[0086]
[Table 3]

[0087]
Thus, it can be seen that in the second embodiment, all of the conditional expressions (1) to (6) are satisfied.
[0088]
4A and 4B are graphs showing various aberrations in the second example. 4A is a diagram showing various aberrations at the lowest magnification (pos1), and FIG. 4B is a diagram showing various aberrations at the highest magnification (pos4). As is apparent from each aberration diagram, in the second example, various aberrations including the infrared region are corrected in a well-balanced manner, and excellent imaging performance is secured.
[0089]
(Third embodiment)
Next, a third embodiment of the present invention will be described with reference to FIGS. FIG. 5 is a diagram showing the lens configuration of the variable magnification imaging lens according to the third embodiment of the present invention and the movement trajectory of each lens group in zooming from low magnification (pos1) to high magnification (pos4).
[0090]
In the variable magnification imaging lens of FIG. 5, the front lens group Gf is arranged in order from the object side, a positive meniscus lens L1 having a convex surface facing the object side, a positive meniscus lens L2 having a convex surface facing the object side, and a convex surface facing the object side. And has a positive refracting power. The rear lens group Gr includes, in order from the object side, a diffractive optical element DOE having a diffractive optical surface, a positive meniscus lens L4 having a concave surface facing the object side, a negative meniscus lens L5 having a concave surface facing the object side, and an object The lens is composed of a cemented lens with a positive meniscus lens L6 with a concave surface facing side, and a three-lens cemented lens with a biconvex lens L7, a biconcave lens L8, and a biconvex lens L9, and has positive refractive power. As the magnification is changed from the low magnification side to the high magnification side, the front lens group Gf and the rear lens group Gr are moved on the optical axis to the object side in a direction to reduce the vertex interval between the front lens group Gf and the rear lens group Gr. I am letting. An aperture stop S is disposed between the front lens group Gf and the rear lens group Gr and in the vicinity of the rear lens group Gr. The aperture stop S moves together with the rear lens group Gr during zooming.
[0091]
Table 4 shows the specifications of each lens in the third embodiment of the present invention shown in FIG. In Table 4, surface numbers 1 to 18 correspond to reference numerals 1 to 18 in FIG. In Table 4, the surface number 6 is the aperture stop S, the surface numbers 8 and 9 are the diffractive optical element DOE, the surface number 10 is the radius of curvature Rr1 of the first surface of the rear lens group first lens L4, and the surface number 11 is the rear. The curvature radius Rr2 of the 2nd surface of the lens group 1st lens L4 is shown. Further, the distance d0 from the object point O to the first surface of the lens system, the surface distance indicated by the surface number 5 (namely, the surface distance between the surface number 5 and the surface number 6) d5, the surface distance indicated by the surface number 19 (namely, the surface). The back focus Bf, which is the surface interval between the number 19 and the image plane I, changes in accordance with the (use) magnification β. Thus, the values that change with zooming are shown for each of the use magnifications pos1 to pos4 in the focusing interval table in Table 4. Furthermore, in Table 4, the values corresponding to the conditional expressions (1) to (6) described above, that is, the condition corresponding values are also shown.
[0092]
[Table 4]

[0093]
Thus, in the third embodiment, it can be seen that all the conditional expressions (1) to (6) are satisfied.
[0094]
FIGS. 6A and 6B are graphs showing various aberrations of the third example. 6A is a diagram showing various aberrations at a low magnification (pos1), and FIG. 6B is a diagram showing various aberrations at a high magnification (pos4). As is apparent from each aberration diagram, in the third example, various aberrations including the infrared region are corrected in a well-balanced manner, and excellent imaging performance is ensured.
[0095]
(Fourth embodiment)
Next, a fourth embodiment of the present invention will be described with reference to FIGS. FIG. 7 is a diagram showing the lens configuration of the variable magnification imaging lens according to the fourth embodiment of the present invention and the movement trajectory of each lens group in zooming from low magnification (pos1) to high magnification (pos4).
[0096]
In the variable magnification imaging lens of FIG. 7, the front lens group Gf is composed of a positive meniscus lens L1 arranged in order from the object side and a convex lens facing the object side, and a cemented lens of a biconvex lens L2 and a biconcave lens L3. , Has a positive refractive power. The rear lens group Gr includes, in order from the object side, a diffractive optical element DOE having a diffractive optical surface, a positive meniscus lens L4 having a concave surface facing the object side, a negative meniscus lens L5 having a concave surface facing the object side, and an object The lens is composed of a cemented lens with a positive meniscus lens L6 with a concave surface facing side, and a three-lens cemented lens with a biconvex lens L7, a biconcave lens L8, and a biconvex lens L9, and has positive refractive power. As the magnification is changed from the low magnification side to the high magnification side, the front lens group Gf and the rear lens group Gf are moved on the optical axis to the object side in a direction to reduce the apex distance between the front lens group Gf and the rear lens group Gr. I am letting. An aperture stop S is disposed between the front lens group Gf and the rear lens group Gr in the vicinity of the rear lens group Gf. The aperture stop S moves together with the rear lens group Gr during zooming.
[0097]
Table 5 shows the specifications of each lens in the fourth embodiment of the present invention shown in FIG. In Table 5, surface numbers 1 to 18 correspond to reference numerals 1 to 18 in FIG. In Table 5, the surface number 6 is the aperture stop S, the surface numbers 8 and 9 are the diffractive optical element DOE, the surface number 10 is the radius of curvature Rr1 of the first surface of the rear lens group first lens L4, and the surface number 11 is the rear. The curvature radius Rr2 of the 2nd surface of the lens group 1st lens L4 is shown. Further, the distance d0 from the object point O to the first surface of the lens system, the surface distance indicated by the surface number 5 (namely, the surface distance between the surface number 5 and the surface number 6) d5, and the surface distance indicated by the surface number 18 (namely, the surface). The back focus Bf, which is the surface spacing between the number 18 and the image plane I, changes in accordance with the (use) magnification β. Thus, the values that change with zooming are shown for each of the use magnifications pos1 to pos4 in the focusing interval table in Table 5. Further, in Table 5, values corresponding to the above-described conditional expressions (1) to (6), that is, condition corresponding values are also shown.
[0098]
[Table 5]

[0099]
Thus, in the fourth example, it can be seen that all the conditional expressions (1) to (6) are satisfied.
[0100]
8A and 8B are graphs showing various aberrations of the fourth example. FIG. 8A is an aberration diagram at the lowest magnification (pos1), and FIG. 8B is an aberration diagram at the highest magnification (pos4). As can be seen from the respective aberration diagrams, in the fourth example, various aberrations including the infrared region are corrected in a well-balanced manner, and excellent imaging performance is ensured.
[0101]
A case where the variable magnification imaging lens according to the present invention is applied to a transparent original reading apparatus will be described. As shown in FIG. 13, the transmissive original reading apparatus includes an illumination light source (1), an original table (2), an optical path bending mirror (3), a variable magnification imaging lens (4) according to the present invention, and a reading unit. And a solid-state image pickup device (CCD or the like) (5). The transparent document reading apparatus having such a configuration is designed to adjust the magnification of the variable magnification imaging lens (4) according to the size of the document when a document (not shown) is set on the surface of the document table (2). The position is moved, the solid-state image pickup element (5) is also moved, and an image is formed with an appropriate conjugate length. The light emitted from the illumination light source (1) illuminates the document set on the surface of the document table (2), and the transmitted light of the document is folded by the optical path folding mirror (3) to obtain variable magnification imaging. An image is formed on the solid-state image pickup element (5) through the lens (4). In accordance with such a reading process, the document table {circle around (2)} moves in the direction of arrow a in the drawing to read image information on the entire surface of the document.
[0102]
【The invention's effect】
As described above, according to the present invention, it is possible to obtain an optical system capable of changing the imaging magnification in which various aberrations including chromatic aberration are corrected in a well-balanced manner to the infrared region with a zoom ratio of 3 or more.
[0103]
Specifically, in the present invention, a variable magnification imaging lens suitable for a color copier, a color facsimile, a color image scanner, a color film scanner, and the like and an image reading apparatus including the same can be realized.
[Brief description of the drawings]
FIG. 1 is a diagram illustrating a lens configuration of a variable magnification imaging lens according to a first example of the present invention.
FIG. 2 (a) is a diagram showing various aberrations at the (most) low magnification (pos1) of the first example, and FIG. 2 (b) is the (most) high magnification (pos4) of the first example. It is an aberration diagram.
FIG. 3 is a diagram illustrating a lens configuration of a variable magnification imaging lens according to a second example of the present invention.
FIG. 4A is a diagram showing various aberrations at the (most) low magnification (pos1) of the second example, and FIG. 4B is the (most) at the (highest) high magnification (pos4) of the second example. It is an aberration diagram.
FIG. 5 is a diagram illustrating a lens configuration of a variable magnification imaging lens according to a third example of the present invention.
FIG. 6A is a diagram showing various aberrations at the (most) low magnification (pos1) of the third example, and FIG. 6B is the (most) high magnification (pos4) of the third example. It is an aberration diagram.
FIG. 7 is a diagram illustrating a lens configuration of a variable magnification imaging lens according to a fourth example of the present invention.
FIG. 8A is a diagram showing various aberrations at the (most) low magnification (pos1) of the fourth embodiment, and FIG. 8B is the (most) high magnification (pos4) of the fourth embodiment. It is an aberration diagram.
FIG. 9 is a diagram showing a sawtooth shape of the diffractive optical surface of the first embodiment.
FIG. 10 is a diagram illustrating a sawtooth shape of a diffractive optical surface according to a second embodiment.
FIG. 11 is a diagram illustrating a sawtooth shape of a diffractive optical surface according to a third embodiment.
FIG. 12 is a diagram illustrating a sawtooth shape of a diffractive optical surface according to a fourth embodiment.
FIG. 13 is a schematic view of a main part when the variable magnification imaging lens according to the present invention is applied to a transparent document reading apparatus.
[Explanation of symbols]
Gf front lens group
Gr Rear lens group
Rr1 radius of curvature of the first surface of the first lens in the rear lens group
Rr2 radius of curvature of the second surface of the first lens in the rear lens group
S Aperture stop
DOE diffractive optical element
O Object point (object surface)
I Image plane
▲ 1 ▼ Illumination light source
(2) Document table
(3) Optical path bending mirror
(4) Variable magnification imaging lens
(5) Solid-state image sensor

Claims (4)

In order from an object consists of a front lens group having positive refractive power, and a rear lens group having positive refractive power, a diffractive optical element having a diffractive optical surface of at least one surface,
While changing the air gap between the front lens group and the rear lens group with zooming from the low magnification side to the high magnification side, both the front lens group and the rear lens group are moved to the object side ,
The diffractive optical element is configured to move together with the front lens group or the rear lens group upon zooming ,
DF represents the amount of movement due to zooming from the lowest magnification to the maximum magnification of the front lens group, DR represents the amount of movement due to zooming from the lowest magnification to the maximum magnification of the rear lens group, and the focal length of the diffractive optical surface is FDOE, When the combined focal length at the minimum magnification of the entire lens system is Fa, the following conditional expression
0.7 <DF / DR <1.0
0.03 <Fa / FDOE <0.1
Satisfied,
The diffractive optical surface has the following conditional expression where hp is the height of the principal ray with respect to the maximum image height incident on the diffractive optical surface, and y is the maximum image height y in all the zooming regions.
| Hp / y | <0.1
A variable magnification imaging lens, which is disposed so as to satisfy the above . 2. The variable magnification imaging lens according to claim 1 , wherein in the rear lens group, the first lens in the rear lens group located closest to the object side has a meniscus lens shape with a concave surface facing the object side. A stop is disposed between the front lens group and the rear lens group, and the distance between the diffractive optical surface and the stop is always kept constant during zooming. The variable magnification imaging lens according to claim 1 or 2 . An image reading apparatus which comprises a variable magnification imaging lens according to any one of claims 1 to 3.

Images