An optical compensator for use in an optical scanning device for scanning a first optical record carrier (3') having an information layer (2') at a first information layer depth d and a second optical record carrier (3') having an information layer (2') at a second, different information layer depth d2. The scanning of the first and second optical record carrier is effected using a scanning spot (16) formed on the information layer by a first radiation beam having a first wavelength and a second radiation beam having a second, different wavelength respectively. The optical compensator includes a substantially circular phase structure having an annular zone arranged in the path of the first radiation beam and the second radiation beam. The annular zone is adapted for imparting a wavefront modification to said first radiation beam causing destructive interference over the area of the scanning spot (16) for radiation incident on said annular zone; and a wavefront modification to said second radiation beam for compensating spherical aberration. A further aspect of the present invention relates to an optical element for defining the numerical aperture of a radiation beam. The optical compensator or optical element can be used for a two- or three-wavelength system. The invention also discloses an optical scanning head, and an optical scanning device using the optical compensator. The invention also relates to an optical scanning head and an optical scanning device.