Apparent pupil displacement in videokeratography.

With videokeratography, the apparent position of the entrance pupil of some eyes is displaced by up to 0.5 mm from the center of the target ring image or the corneal map. This displacement occurs because of the alignment system and is not affected significantly by the corneal central radius of curvature, peripheral asphericity, or other shape parameters. It is determined primarily by angle lambda between the line of sight and the pupillary axis and is reduced when the real pupil is decentered nasally. The displacement of the entrance pupil position occurs in all commercial videokeratographs and is responsible for the difference in the apparent position of the entrance pupil when observed directly by the examiner's eye, in refractive surgery, and when measured by videokeratography. This difference also demonstrates that the line of sight should represent the primary reference axis for videokeratography and photorefractive surgery rather than the vertex normal or the visual axis.