US Patent 7301709 Imaging optical system with focusing function

An imaging optical system is provided and has a focusing section including a cemented lens (first and second lenses), which includes first and second fluid materials having different refractive indices from each other; and whose cemented surface shape can be electrically controlled. A refractive index Nd1 of the first fluid material, a refractive index Nd2 of the second fluid material, a minimum value RA of a radius of curvature of the cemented surface of the cemented lens, a focal length fa of the whole cemented lens, and a focal length f of the whole lens system satisfy the conditions specified in the specification.