Optical recording/reproduction devices for large-capacity optical discs such as Blu-ray discs (BD) have become common in recent years.  In an optical recording/reproduction device for a BD, in comparison with an optical recording/reproduction device for a DVD, the spot of the laser beam that illuminates the information recording surface of the optical disc is smaller, the wavelength of the laser light is shorter, and the numerical aperture (NA) of the objective lens is greater, and for these and other reasons, the effect of spherical aberration must be considered.  It is therefore necessary to control the amount of spherical aberration in an optical recording/reproduction device for a BD (see, for example, Patent Reference 1).  Patent Reference 1 discloses a method that controls the amount of spherical aberration so as to maximize the amplitude of a tracking error signal. 
Due to demands for still greater capacities, multilayered BDs having multiple recording layers have become common.  In recording or reproducing data on a multilayered BD, processing occurs for performing an interlayer jump from one recording layer to another, and depending on the distance between the layers, the optimal amount of spherical aberration in each recording layer may differ.  Methods of controlling the optimal amount of spherical aberration in each recording layer have therefore been proposed (see, for example, Patent References 2 and 3). 
Patent Reference 2 discloses a method of calculating spherical aberration offsets between the recording layers, based on the S-shaped waveform of a focus error signal detected when the objective lens is displaced in the focusing direction. 
Patent Reference 3 discloses a method that determines the amount of spherical aberration that maximizes the amplitude of the S-shaped waveform of the focus error signal detected when the objective lens is displaced in the focusing direction and sets that amount of spherical displacement, multiplied by a correction coefficient, as the optimal amount of spherical aberration for each recording layer.