Theoretical and experimental techniques are developed to study three common approaches to focus error signal generation for optical data storage applications. Specifically, the astigmatic, obstruction knife edge (a.k.a. shaded aperture), and critical angle prism systems are studied. These techniques are then applied to a new focus detection system, based on a lateral shearing interferometer, developed by this author. The sensitivity of a given optical system to the primary design point, alignment errors, aberrations and optical recording medium topology are presented. A new approach to constructing the focus error signal for the astigmatic and interferometric techniques is presented which greatly reduces the alignment sensitivity.