US Patent 7898748 Optical recording/reproducing apparatus

An optical recording/reproducing apparatus is arranged so that, upon recording or reproducing a recording medium having a plurality of recording layers, the gap of the lens groups at the time of forming a converged light spot on this first recording layer is DIS (1), the gap of the lens groups at the time of forming the converged light spot on the N-th layer (the furthest layer from the surface of the recording medium) is DIS (N) and the neutral point of the spherical-aberration correcting mechanism is set at a position satisfying the following expression: lens group gap dst (3)=((DIS (1)+DIS (N))/2.