PROBLEM TO BE SOLVED: To provide an imaging optical system for endoscopes capable of changing over between a wide-angle observation state for simultaneously observing an object in front and an object substantially lateral thereto and a telescopic observation state for observing only the object in front and capable of favorably correcting chromatic aberration of magnification in either state.SOLUTION: This imaging optical system comprises: in the order from an object side, a first group 1 consisting of a single lens with a concave facing an image side and having a negative refractive power a second group having a catadioptric lens and having the negative refractive power a third group having an aperture stop and having a positive refractive power and a fourth group having a cemented lens including a positive lens and having the positive refractive power. The third group is moved between two home positions on an optical axis along the optical axis to change over between the wide-angle observation state and the telescopic observation state and satisfy the following conditional expressions: 1<&nud4p/&nud3<2.4, and &nud3<51, where &nud4p is Abbe number of the positive lens and &nud3 is Abbe number of the third group.COPYRIGHT: (C)2012,JPO&INPIT【課題】 前方の物体及び略側方の物体の同時観察を行う広角観察状態と前方の物体のみの観察を行う望遠観察状態とを切り替えることができ、いずれの状態においても倍率色収差を良好に補正することのできる内視鏡用撮像光学系を提供すること。内視鏡用撮像光学系を提供すること。【解決手段】 物体側から順に、像側に凹面を向けた単レンズからなり負の屈折力を持つ第1群と、反射屈折レンズを有し負の屈折力を持つ第2群と、開口絞りと、正の屈折力を持つ第3群と、正レンズを含む接合レンズを有し正の屈折力を持つ第4群とにより構成され、前記第3群を、光軸上の2箇所の定位置に光軸に沿って移動させることにより、広角観察状態と望遠観察状態とを切り替え、以下の条件式を満足する。1 < νd4p/νd3 < 2.4νd3 < 51ただし、νd4pは前記正レンズのアッベ数、νd3は第3群のアッベ数である。【選択図】 図3
