PURPOSE: To reduce a coma aberration and astigmatism and to reduce the deterioration of a picture due to a chromatic aberration by using a lens system with positive refracting power as a 1st group lens arranged on the outermost object side and obtaining a ≤40° field angle.

CONSTITUTION: The 1st group lens I on the outermost object side which constitutes a lens system is given positive refracting power and its field angle is ≤40°. This objective lens having the small field angle can be made more compact by being constituted as a telephoto type than as a retrofocus type. When this is varied in power by employing an adapter system, the field angle when only a master lens M is used never becomes narrow by using the 1st group lens I with positive refracting power unlike adapter system variable power where the master lens M is arranged on the picture side of the retrofocus type lens system. Thus, the lens is composed of the 1st group lens I with positive refracting power and a 2nd group lens II with negative refracting power, and then the coma aberration generated by the 1st group positive lens I is compensated by the 2nd group negative lens II. Consequently, the constitution is compact, the coma aberration and astigmatism are small, and the detection of an picture due to the chromatic aberration is reduced.