Image Number 2 for United States Patent #6212149.
An optical pickup includes a photo-detector. The photo-detector is structured by light receiving elements on one of which a blind zone is formed. If a relationship between an output of the light receiving element formed with a blind zone and a lens position is displayed as a graph on a measurement signal output device, a point that a beam converges on the blind zone, i.e., a point that a spot size assumes a minimum, is displayed as an edge. In the photo-detector, a region for receiving a sub-beam on light shield side, i.e., one sub-beam having a grater spot size, has a width broader than a width of a region for receiving a sub-beam on an aperture side, i.e., another sub-beam smaller in spot size.