Image optical system including a non-spherical single lens

摘要: The imaging optical system according to the present invention comprises a non-spherical single lens having first surface of shape in which radius curvature becomes gradually greater away from optic axis and planar second surface, parallel flat plate disposed adjacent as required. satisfies following conditions: ##EQU1## where f is focal length lens, d on-axis thickness n refractive index NA numerical aperture r1 vicinity axis, t plate.