A liquid crystal lens element capable of finely adjusting focal length according to the magnitude of applied voltage, and an optical head device employing such a liquid crystal lens element, are provided. The thickness irregularity of a transparent layer can also be calculated from the detected aberration amount. The low current condition is characterized by thresholds corresponding to the peak level and a valley level of the channel current over a switching time period. The microcontroller receives the first and second current sensing signals to determine a loading state. A two-cone drill bit having a bit body formed from two leg sections and two spacing members; methods of manufacturing a two-cone drill bit; and methods of improving hydraulics of a two-cone drill bit are disclosed. In addition, the word line driver has a level shift circuit for shifting a voltage level of the word line, and the level shift circuit has an input end connected to the second circuit of the address decoder.