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

AijAijThe multi-wavelength lens for condensing a plurality of kinds of monochromatic light by refraction is disclosed. The lens comprises 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. At least half of the step portions satisfy a following formula when a minimum value and a maximum value of Aij for each wavelength λi are MIN(Aij) and MAX(Aij), respectively: MAX()/MIN()