Although angle of view being wide angle above the 140 , it actualizes the revision of distortion and the enlargement of observation depth simultaneously, even in the image pickup element of the high pixel makes ideally applicable. From object side the pre-group (FG), the brightness contraction (S) and the group after the righteousness (BG) it consists of in order, the pre-group (FG), as 1 aspects are formed at least with the aspheric surface, most it is satisfied the below-mentioned conditional expression the negative 1st lens which (1) (L1) it has on object side, the objective optical system where angle of view is above the 140 it offers (1). As for r1 the 1st lens (L1) radius of curvature of object side, as for r2 the 1st lens (L1) it is radius of curvature of image side. (1) 0 r2/r1<0.24画角が140°以上の広角でありながら、ディストーションの補正および観察深度の拡大を同時に実現し、高画素の撮像素子にも好適に適用可能にする。物体側から順に前群(FG)、明るさ絞り(S)および正の後群(BG)からなり、前群(FG)は、少なくとも1面が非球面で構成されるとともに、最も物体側に下記の条件式(1)を満足する負の第1レンズ(L1)を備え、画角が140°以上である対物光学系(1)を提供する。r1は第1レンズ(L1)の物体側の曲率半径、r2は第1レンズ(L1)の像側の曲率半径である。(1)・・・0≦r2/r1<0.24
