Patent ID: 6901045

Claim:
A focus error signal production method for irradiating light upon an optical recording medium to obtain focus information of returning light from the optical recording medium, comprising the step of: obtaining focus information of the returning light from the optical recording medium using an optical lens which includes a substrate made of an optical material that has a continuous flat face on a first side through which an axis in a focus direction of said optical lens passes and a convex element formed integrally with the substrate and having a convex curved face that protrudes from a second side of said substrate opposite from said first side so as to have a function as an optical lens, the convex element being configured such that a focal length on a first cross section including the axis in the focus direction of the optical lens and a focal length on a second cross section perpendicular to the first cross section and intersecting with the first cross section along the axis in the focus direction are different from each other.