of the image point relative to the optical axis 2 of the overall system before the off centering is given by if a power u03c6 of the aberration compensation lens unit l to a reference wavelength u03bb 0 is essentially zero u03c6 u2245 0 the focusing magnification u03b2 of the aberration compensation lens unit l to the reference wavelength u03bb 0 is essentially 1 and the movement distance u03b4y of the image point is essentially zero u03b4y u2245 0 if the focusing magnification u03b2 is set to u03b2 gt 1 for a wavelength u03bb 1 u03bb 1 gt u03bb 0 and to u03b2 lt 1 for a wavelength u03bb 2 u03bb 2 lt u03bb 0 and if the movement distance u03b4h of the aberration compensation lens unit l meets u03b4h gt 0 the displacement ay for the wavelength u03bb 1 is u03b4y lt 0 as shown in fig2 b and the displacement u03b4y for the wavelength u03bb 2 is u03b4y gt 0 as shown in fig2 c thus a diversing system is provided for the wavelength u03bb 1 which is longer than the reference wavelength u03bb 2 and a conversing system is provided for the wavelength u03bb 2 which is shorter than the reference wavelength u03bb 0 accordingly as the aberration compensation lens unit l is off centered while its center axis 3.