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Our aim was to analyze viewing distance for smartphone users (aged 18-45 y.) in terms of passive or active task, relation to heterophoria, type of refractive error and smartphone font size. Participants were asked to read out loud text message (passive task) and afterwards rewrite the same text and send back (active task). For the text message we used sentence consisting of 23 words and 200 characters (with spaces). Participants used their own smartphones (font size monitored). For both passive and active task we measured task time and viewing distance at the end of the task. We found significantly shorter viewing distance for digital active task compared to passive task (29.3 ± 4.7 and 32.3 ± 6.0 cm) and also for digital passive task compared to hardcopy passive task (32.3 ± 6.0 and 34.4 ± 5.9 cm). There was no difference between viewing distance with and without low plus lenses (low add) for digital active task (29.9 ± 5.4 and 29.3 ± 4.7 cm) and for digital passive task (31.4 ± 6.4 and 32.3 ± 6.0 cm). Viewing distance and reading/writing speed was not influenced by type of refractive error and heterophoria type. The interaction between task type and heterophoria type was not significantly associated neither with viewing distance (P = 0.77), nor relative viewing distance (P = 0.54). Writing speed decreased significantly with age (P < 0.001), while reading speed was not influenced by the age (P > 0.05). The explanation, why some people prefer closer viewing distance when using smartphone, seems to be more related to task type and relatively shorter length of forearm not type of refractive error or near heterophoria type.
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