title: Optical characteristics of Alvarez variable-power spectacles.
abstract: To establish the optical performance of currently available Alvarez variable-power spectacles in relation to their possible utility for ametropes and presbyopes.
  
  
  
  Two commercial designs of variable-power (variable-focus) spectacles were studied: FocusSpecs and Adlens Adjustables. Using focimetry, power and prism at three different power settings were measured across the aperture of the lenses of each design and local optical quality was explored in more detail using interferometry.
  
  
  
  In general, equivalent-sphere powers at all lens settings were adequately stable across the central aperture (about 20 mm diameter) of the lenses, with prism changes following the Prentice rule. Some astigmatism (generally ≤0.5 DC over the central area) and higher-order aberration (comparable to the normal wavefront aberration of the eye) were found, the latter increasing towards the edge of the useable lens area.
  
  
  
  Within their designed power ranges, optical performance of currently-available Alvarez variable-power spectacles appears to be adequate to correct the distance and near vision of spherical ametropes.
authors.affiliation: Division of Pharmacy and Optometry, Faculty of Biology, Medicine and Health, University of Manchester, Manchester, UK.
authors.forename: Hema
authors.initials: H
authors.lastname: Radhakrishnan
authors.affiliation: Division of Pharmacy and Optometry, Faculty of Biology, Medicine and Health, University of Manchester, Manchester, UK.
authors.forename: W Neil
authors.initials: WN
authors.lastname: Charman
mesh_terms: D004867:Equipment Design; D005139:Eyeglasses; D006801:Humans; D055095:Optics and Photonics / Q000379:methods*; D012029:Refraction, Ocular; D012030:Refractive Errors / Q000503:physiopathology / Q000628:therapy*
keywords: Alvarez lenses; focimetry; interferometry; presbyopia; variable-power spectacles