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Clinical studies: visual acuities beyond Snellen
  1. L T Lim1,
  2. D G Frazer2,
  3. A J Jackson2,3,4
  1. 1
    Tennent Institute of Ophthalmology, Glasgow, UK
  2. 2
    Royal Victoria Hospital, Belfast, UK
  3. 3
    University of Ulster, Coleraine, UK
  4. 4
    Queen’s University of Belfast, Belfast, UK
  1. L T Lim, Eye Department, Gartnavel General Hospital, 1053 Great Western Road, Glasgow G12 0YN, UK; likthai{at}yahoo.com

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Visual acuity is a measurement of the eyes’ ability to resolve spatial detail. Within clinical settings, it has traditionally been assessed using test charts incorporating optotypes.1 In the UK, data are generally specified as an “Acuity Fraction,” although in Europe, decimal conversion is commonplace. The numerator indicates the test distance and the denominator the distance at which the test letter is positioned in order to subtend an angle of 5 minutes of arc at the eye.2 This fraction is recognised universally as a measure of “Visual Acuity.” When assessed using conventional optotypes, presented on a Snellen Chart at the standard 6 m testing distance, measurements range from 6/4 to 6/60. Poorer vision can be quantified by reducing the working distance, thus extending …

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