Apparatus for correcting spherical aberration of light reflected from a surface of an information-bearing layer of an optical disk and traversing through a substantially transparent substrate layer, overcoating the information layer, prior to traversal of a multi-element objective lens means, uses a correction lens assembly having several lens elements, with a variable air gap between the adjacent surfaces of a sequential pair of the lens elements. Information about the nominal thickness of an optical disk from which optical energy is then being reflected is obtained, and used to adjust the thickness of the air gap, to correct the additional spherical aberration caused by the change in covering substrate thickness and change the definition of the corrected spot to approach the definition of a spot obtained with a covering substrate of exactly the nominal value.