We wished to investigate how normal subjects with optically induced refractive error performed when reading Sloan crowded single letters (SCSL). The 10 Sloan letters are used in the ETDRS Visual Acuity charts. While linear presentations of the letters used in the ETDRS visual acuity chart have been extensively studied, the effect, if any, of single letter presentation with flanking "crowding bars" has not. These contour interaction bars (with 1.5 MAR white space between the letters and bars) are intended to increase the difficulty of reading letters so as to more closely resemble the difficulty of linear presentations. We studied the ability of a group of 10 normal subjects to read crowded single letter presentations of the 10 letters used in the ETDRS wall charts. The frequency of recognition of the individual letters in discriminant lines was measured, as was the threshold acuity for each of the induced refractive errors. Results suggest that crowded, single letter presentations are recognized over a range of refractive errors in a manner similar to the recognition of linear presentations of the same letters. Letters with diagonal strokes (Z,V,N) remain relatively easy to recognize, while round letters (C,O,D) remain most difficult. Copyright 2000 Optical Society of America

© 2001 Optical Society of America

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