title: The effects of optical defocus on the legibility of the Tumbling-E and Landolt-C.
abstract: BACKGROUND
  The Tumbling-E and Landolt-C are both used to test visual acuity in young, illiterate, or non-English-speaking patients. In this study, we sought to determine how optical blur affects visual acuity thresholds measured with both of these optotypes.
  
  
  METHODS
  Ten subjects were tested with 4-position Tumbling-E and Landolt-Cs using the method of constant stimuli. The subjects were tested wearing their habitual corrections and with +1.50 D spherical and cylindrical lenses to simulate myopic refractive errors in the optical blur test conditions.
  
  
  RESULTS
  Visual acuity thresholds for the Tumbling-E were slightly better for all test conditions, while the largest threshold difference between the test letters was found for the simulated against-the-rule astigmatism. For both test letters, vertical orientations were more difficult to resolve than horizontal. Also, the slopes of the psychometric functions were steeper for the Tumbling-E than for the Landolt-C.
  
  
  CONCLUSIONS
  The Tumbling-E can be considered comparable with the Landolt-C except for the simulated against-the-rule-astigmatism test condition. Although the ranges are quite large, the average psychometric functions of the Landolt-Cs are flatter, which gives rise to the possibility that more confidence can be placed in the visual acuity thresholds obtained with the Tumbling-E.
authors.affiliation: NOVA Southeastern University, College of Optometry, Fort Lauderdale, Florida 33328-2018, USA. lreich@hpd.acast.nova.edu
authors.forename: Lewis N
authors.initials: LN
authors.lastname: Reich
authors.forename: Michele
authors.initials: M
authors.lastname: Ekabutr
mesh_terms: D001251:Astigmatism / Q000503:physiopathology; D006801:Humans; D009216:Myopia / Q000503:physiopathology; D011932:Reading; D012684:Sensory Thresholds; D014787:Vision Tests / Q000379:methods*; D014792:Visual Acuity