title: Determination of the influence of effectivity upon residual astigmatism.
abstract: OBJECTIVE
  A computing scheme is described which allows determination of the astigmatic contribution of ocular surface effectivity towards residual astigmatism.
  
  
  METHODS
  This involves paraxial raytracing through astigmatic surfaces at random axes and applies the principle of astigmatic decomposition. Calculations are shown for averaged data from 66 normal right eyes. Frequency distribution graphs demonstrate individual variations.
  
  
  RESULTS
  The averaged ratio of corneal thickness:anterior chamber:lens thickness cylinder power contributions due to effectivity (1:5:17) did not match the ratio of their respective intraocular distances (1:7:7); a disproportionate amount of astigmatism arose from lens thickness effectivity. Although previous research has revealed that results for individual eyes are prone to accummulative experimental errors, frequency distribution graphs indicate that effectivity predominantly yelds direct astigmatism (axis 180 degrees +/- 22.5 degrees).
  
  
  CONCLUSIONS
  This computing scheme offers a means of examining the functional ocular morphology of astigmatic eyes.
authors.affiliation: Department of Vision Sciences, Aston University, Birmingham, UK.
authors.forename: M C
authors.initials: MC
authors.lastname: Dunne
authors.forename: M E
authors.initials: ME
authors.lastname: Elawad
authors.forename: D A
authors.initials: DA
authors.lastname: Barnes
mesh_terms: D000293:Adolescent; D000328:Adult; D000867:Anterior Chamber / Q000473:pathology; D001251:Astigmatism / Q000175:diagnosis*; D003196:Computer Graphics; D003315:Cornea / Q000473:pathology*; D003626:Data Display; D005260:Female; D006801:Humans; D007091:Image Processing, Computer-Assisted; D007908:Lens, Crystalline / Q000473:pathology*; D008297:Male; D009885:Ophthalmology / Q000379:methods*; D012029:Refraction, Ocular; D015203:Reproducibility of Results