title: Optical defocus: differential effects on size and contrast letter recognition thresholds.
abstract: OBJECTIVE
  To determine if optical defocus produces a greater reduction in visual acuity or small-letter contrast sensitivity.
  
  
  METHODS
  Letter charts were used to measure visual acuity and small-letter contrast sensitivity (20/25 Snellen equivalent) as a function of optical defocus. Letter size (acuity) and contrast (contrast sensitivity) were varied in equal logarithmic steps to make the task the same for the two types of measurement.
  
  
  RESULTS
  Both visual acuity and contrast sensitivity declined with optical defocus, but the effect was far greater in the contrast domain. However, measurement variability also was greater for contrast sensitivity. After correction for this variability, measurement in the contrast domain still proved to be a more sensitive (1.75x) index of optical defocus.
  
  
  CONCLUSIONS
  Small-letter contrast sensitivity is a powerful technique for detecting subtle amounts of optical defocus. This adjunctive approach may be useful when there are small changes in resolution that are not detected by standard measures of visual acuity. Potential applications include evaluating the course of vision in refractive surgery, classification of cataracts, detection of corneal or macular edema, and detection of visual loss in the aging eye. Evaluation of candidates for occupations requiring unique visual abilities also may be enhanced by measuring resolution in the contrast domain.
authors.affiliation: Aircrew Health and Performance Division, U.S. Army Aeromedical Research Laboratory, Fort Rucker, AL 36362-0577.
authors.forename: J
authors.initials: J
authors.lastname: Rabin
mesh_terms: D000328:Adult; D015350:Contrast Sensitivity / Q000502:physiology*; D005556:Form Perception / Q000502:physiology; D006801:Humans; D012684:Sensory Thresholds / Q000502:physiology*; D014786:Vision Disorders / Q000503:physiopathology*; D014787:Vision Tests; D014792:Visual Acuity / Q000502:physiology*