The present invention is directed to a compact lens system equally operable in both air and water, comprising a first lens group having a front surface directly facing the air or water medium, the center of the curvature of the front surface being located approximately at the entrance pupil of the lens system, and a second lens group located at the image side of the first lens group with an air space formed between the first and second lens groups. The air space and design parameters being selected in the lens system so that the field curvature of the lens system is mainly changed without any substantial change in other aberrations when the air space is changed, wherein the air space is changeable by a predetermined value in response to the change between use in air and use under water for maintaining a relatively flat image surface.