A method used in an optical storage device having a pickup head. The pickup head includes an object lens, a first and a second light source. The object lens focuses the laser light generated by the first or the second light source onto a first or a second focal point. A driving control effort is used to change the location of the object lens. The method includes the following steps: providing the driving control effort to move the object lens; measuring a first parameter when the first focal point passes through a predetermined layer of an optical disc; measuring a second parameter when the second focal point passes through the predetermined layer of the optical disc; and (d) determining the relationship between the difference of object lens location and the driving control effort according to the predetermined distance, the first parameter, and the second parameter.