PURPOSE: To make a main lens alone usable as a telescope for high magnification visual observation by removing an auxiliary lens and to use the entire system as a bright photographing lens having a wide angle of view by satisfying prescribed conditions in a lens system consisting of a main lens group and an auxiliary lens group.

CONSTITUTION: In a lens system of 2 group 4 element (each group 2 elements) constituted of a main lens group and an auxiliary lens group, the conditions of the separately described 13 equations are satisfied among the radii Ri of curvature of each lens face, interface space di, each focal lenght F, and glass material Ni (Ndi, νi). Thereby, the main lens group alone by removing the auxiliary lens is usable as a telescope for high magnification visual observation, and the composite optical system of the main lens and the auxiliary lens is usable as a bright photographing lens having a wide angle of view.