title: Theoretical effect of changes in entrance pupil magnification on wavefront-guided laser refractive corneal surgery.
abstract: OBJECTIVE
  To explore theoretically the effect on the correction of higher order aberrations of changes in the magnification between the aperture stop (iris) and entrance pupil of the eye, following myopic excimer laser ablation.
  
  
  METHODS
  Using a simple schematic eye model, paraxial calculations were made of the position and magnification of the entrance pupil of the eye as a function of the power of a myopic photorefractive keratectomy correction.
  
  
  RESULTS
  Corneal flattening following myopic corneal ablation results in a reduction in the magnification between the aperture stop (iris) and entrance pupil. This implies that an ablation designed to correct higher order errors on the basis of the preoperative wavefront aberration across the entrance pupil will result in an incomplete correction. Taking into account the fact that the total ocular aberration depends on the combined effects of all optical components of the eye, which are distributed in depth, rather than being associated simply with the anterior surface of the cornea, the exact effects depend on the methods used to measure the aberration and the distribution of the total aberration between the different components of the eye. The errors in correction increase with the magnitude of the myopic correction.
  
  
  CONCLUSIONS
  To minimize the postoperative higher order aberrations in higher amounts of myopia, it may be desirable to remeasure them after a first ablation to correct the second-order refractive errors and then to carry out a second ablation to correct the higher order aberrations.
authors.affiliation: Department of Optometry and Neuroscience, University of Manchester, Manchester, United Kingdom. neil.charman@manchester.ac.uk
authors.forename: William N
authors.initials: WN
authors.lastname: Charman
authors.forename: David A
authors.initials: DA
authors.lastname: Atchison
mesh_terms: D003315:Cornea / Q000601:surgery*; D019781:Corneal Topography / Q000379:methods*; D006801:Humans; D054018:Lasers, Excimer; D008962:Models, Theoretical; D009216:Myopia / Q000601:surgery*; D018903:Photorefractive Keratectomy / Q000379:methods*; D011680:Pupil / Q000502:physiology*; D012029:Refraction, Ocular; D014792:Visual Acuity