Imaging optical system

An imaging optical system (10) having a focal length (f) and a maximum imagewise numerical aperture. The system combines diffractive and refractive optical elements such that aberrations, including axial chromatic aberration, are well-corrected over a large wavelength range from the visible into the infrared. The system comprises, objectwise to imagewise along an optical axis (A), a front lens group (GF) having at least one refractive optical element, a diffractive optical element (DOE) provided with a diffractive surface (D) having positive diffractive power and a focal length (fDOE). The system also includes a rear lens group (GR) having at least one refractive optical element. The system has an amount of spherical aberration at the maximum imagewise numerical aperture, is measured with respect to e-line wavelength light, in the limit as the positive diffractive power of the diffractive surface approaches zero, of RSA. The system also has a maximum amount of axial chromatic aberration of the imaging optical system over a wavelength range of 400-850 nm, as measured with respect to e-line reference wavelength light, of LA. Further, the following conditions, as well as several others, are preferably satisfied:-0.01