Washington: A new study has suggested that smart phone users reading text messages and internet pages hold their devices at a closer distance than they would for printed text -- which may have important implications for prescribing vision correction.

According to the study by Mark Rosenfield and colleagues of SUNY State College of Optometry, New York, optometrists should pay attention to the "working distance" at which patients hold their smart phones and perform appropriate testing at those distances.

The researchers evaluated viewing distance and font size in smart phone users. While wearing their usual vision correction (glasses or contact lenses), 129 smart phone users were asked to demonstrate how they would hold their device while reading a text message or a typical Internet page.

The average font size was comparable to that of newspaper print for text messages, but somewhat smaller for the Internet page. The average working distance for text messages was 36 centimeters (14.2 inches). This was closer than the typical near working distance of 40 centimeters (15.7 inches) when reading printed text. The average working distance when viewing a web page on a smart phone was even shorter: 32 centimeters (12.6 inches).

Holding smart phones at such close distances could place increased demands on the eye's ability to correct for distance (accommodation) and coordination between eyes (vergence), compared to the distances typically used for reading printed text. Over time, this could lead to symptoms such as eyestrain and headaches.

The new study suggested that patients tend to use handheld devices at closer distances compared to printed materials.

Smart phones "may present a variety of visual demands that are significantly different in terms of working distances, gaze angle, and text sizes," said Rosenfield. The study has been published in the Optometry and Vision Science.