Multi-wavelength lens, and optical system, optical head and optical disc apparatus using the lens

A multi-wavelength lens for condensing a plurality of kinds of monochromatic light by refraction, comprising:

• a common use area for all monochromatic light on at least one lens surface, the area sectioned into a plurality of aspherical zones each of which having a different refractive power;
• and
• step portions, each of which formed between adjacent aspherical zones of the plurality of aspherical zones and having a step height Dj (j=1, 2, 3, 4, and so on, in order of closeness to a lens optical axis) in a direction parallel to the lens optical axis,
• wherein at least half the number of the step heights of the step portions satisfy MAX(Aij)/MIN(Aij)<3,
• wherein MIN(Aij) and MAX(Aij) are minimum and maximum values of phase shift amount Aij derived from said step height for each wavelength λi, respectively, Aij=absolute (Bij−mij), Bij=absolute(Dj))*(ni−1)/λi−(NAij2)*K/λi, ni is a refractive index of a lens for a wavelength λi, mij is an integral number closest to Bij, and NAij is a numeral aperture (NA) of each of the aspherical zones defined by each of the step portions for a wavelength λi:
• K=0.0004 mm when NAij <0.55
• K=0.0005 mm when NAij >=0.55.