Abstract: The invention relates to an optical device in which a spherical aberration is detected. The optical device comprises means (206, 208) for focussing a first radiation beam (203) having a first numerical aperture and a second radiation beam (204) having a second numerical aperture lower than the first numerical aperture, on an information carrier (200). The optical device further comprises means (211) for detecting a first focus error signal corresponding to the first radiation beam and a second focus error signal corresponding to the second radiation beam. In order to measure the spherical aberration of the first radiation beam, due to a variation of the cover layer thickness of the information carrier, the optical device comprises means (212) for measuring a spherical aberration of the first radiation beam from the first and second focus error signals.