Choosing viewing distances according to print size in low vision and normal subjects

Abstract

Abstract: : Purpose: When reading small print, the viewing distance is often reduced in order to increase the angular size of the reading material. It is well known that when the angular size of print is close to the limit of resolution, reading speed slows. Accommodation will impose a limit on the close range of viewing distance. Knowing about the angular size and reading efficiency levels that subjects choose, may influence decisions about the use of large print materials or magnifiers. Methods: We have monitored viewing distances and reading speeds as subjects have read aloud from word reading charts and sentence reading charts on which the size ranges from 5M to 0/0.05M. Subjects were free to change their observation distances as they read the charts going from large to small, and from small to large. The normally–sighted subjects were 8 adult college students. There were also 7 visually impaired 4th grade schoolchildren who were visual readers and they also read samples of familiar classroom materials at 3 enlargement levels Results: The normally sighted subjects showed a pattern of adjusting their viewing distance, such that the smallest angular size adopted averaged 7.8 minarc (+/–1.2) and the minimum viewing distances adopted averaged 15cm (+/–2.2), and the smallest print read averaged 0.17M (+/– 0.04). Within the visually impaired schoolchildren, there was great diversity in the vision and the reading characteristics. Three students had their closest reading distances at 9–10 cm and smallest size of print read ranged from 1.25M to 0.25 M. For the other four, the closest reading distance ranged from 3–6 cm and the smallest print read ranged from 1.6M to 0.25M. They tended to change viewing distance to maintain a relatively constant angular size. Most adopted relatively longer viewing distances when reading familiar classroom materials. Conclusions: For both groups, subjects chose to work at distances that provide themselves with a visual acuity reserve of about 2 fold. The visually–impaired fourth graders generally adopted close working distances that appear to at or closer than their limit of accommodation. Even when striving to read small print, the young adult subjects change their viewing distance to maintain an angular size that is substantially larger than their threshold size, and they do not read at their acuity limit. Reading speed only reduced marginally as the very small print sizes were being read.

Keywords: reading • visual acuity • aging: visual performance