When reading from an electronic screen, many individuals report symptoms such as tired eyes or eye strain. To minimize these symptoms, a 3× acuity reserve has been suggested, i.e., the minimum print size should be at least 3 times larger than the size of the letters at the reader’s acuity limit. This study evaluated whether the 3× rule is appropriate, or if an alternative relationship between visual acuity and letter size would be preferable. The experiment was performed on 25 visually-normal subjects who viewed a series of random words on a computer monitor. The threshold distance (T) at which the text could be resolved was determined. Both reading speed and accuracy were measured during a 10min task performed at T and at 0.5T, 0.33T and 0.25T. In a second study, the procedure was repeated at 0.5T and 0.33T for 1h. Immediately following the 1h trial, subjects completed a questionnaire concerning symptoms experienced during the task. The mean T for the 10min trial was 131.4cm. While the mean reading speeds for the 0.5T, 0.33T and 0.25T conditions were significantly different from the 1T condition, they were not significantly different from each other. For the 1h trial, reading speed at 1T was significantly different from both the 0.5T and 0.33T conditions, but no significant difference was observed between the 0.5T and 0.33T trials. A significant increase in the total post-task symptoms was found after reading at the threshold distance (T), compared with 0.5T and 0.25T, but no significant difference in symptom score was found between the 0.5T and 0.25T conditions. Accordingly, based on a change in viewing distance (and angular print size) in young, visually-normal subjects, a 2× rule may be appropriate, i.e., for sustained comfortable reading, the text size should be at least twice the individual’s visual acuity. However, higher values may be necessary for older subjects, or individuals with visual abnormalities. 
Is the 3× reading rule appropriate for computer users?