Why Do People Read So Close? The Trade-off Between Clearer and Bigger

Abstract

Abstract: : Purpose:Children and young adults often hold text much closer to their faces than the 14 to 16 inches generally recommended, some as close as 7 inches. Yet it is doubtful that these young people accommodate adequately to bring this near text into clear focus. Young readers appear to make a trade-off between larger text (by holding the text closer) and image clarity, some emphasizing text size while others emphasize image clarity. We hypothesized that ocular aberrations, their changes with accommodation, and accommodative gain may underlie the difference between people who choose closer and bigger rather than farther and clearer. Methods: We have simulated the effect of text size, reading distance, accommodative lag, wavefront aberrations and aberration changes induced by accommodation to show how these factors combine to determine legibility and the spatial spectra of the text’s retinal image. The spatial frequency spectra of 9, 10 and 12 point text held at different distances were analyzed using CT View (Sarver, Inc). The spatial spectra of its retinal image was then calculated based on different amounts of defocus and the ocular aberrations of individual eyes selected from a large group between 10 and 29 years of age who were tested in either Beijing or Boston (He et al, 2002). Results:Moving text close to eyes with little aberration and a high accommodative gain simply increases its size. If accommodative gain is low (< 0.7), legibility is reduced for people with little aberration because defocus induces retinal image degradation that is greater than the benefit of larger text. If a person has high amounts of ocular aberration, the retinal image of text is degraded at 14 inches. Moving the text closer to the face even with relatively low accommodative gains can increase legibility due to relatively little additional image degradation relative to the increase in text size. Conclusions: Since these font sizes are 3 or more times the size of visual acuity threshold letters at 14 inches, size and therefore distance should be of little importance to a reader with small amounts of ocular aberrations and accommodative lag. Young people with high aberrations may bring text close to the face to compensate for inherent image degradation since the additional blur induced by accommodative defocus is relatively small. This may explain why some young myopes tend to read at very close distances.

Keywords: reading • optical properties • accommodation