the invention relates to an imaging optical system that is of a small - format size yet capable of easily securing a back focus , and an imaging system that incorporates the same . the imaging optical system comprises only two lenses , specifically , a first meniscus lens l 1 convex on its object side and a second lens l 2 having positive refracting power and a convex surface configured such that the absolute value of the curvature of its object side - surface is larger than the absolute value of the curvature of its image side - surface . an aperture stop s is located on the object side of an image - formation lens arrangement . condition concerning the focal length ratio between the first meniscus lens l 1 and the second lens l 2 and condition concerning the shape factor of an air lens between the first meniscus lens l 1 and the second lens l 2 are satisfied .