Study evaluates viewing distance and font size in smart phone users

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, reports a study in the July issue of Optometry and Vision Science, official journal of the American Academy of Optometry. The journal is published by Lippincott Williams & Wilkins, a part of Wolters Kluwer Health.

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

Font Size versus Viewing Distance in Smart Phone Users

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 researchers then measured the size of the text on the handheld devices. The goal was to find out whether the working distance at which users held their phones was appropriate for the font size on the devices.

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.

Typically, optometrists follow a '1, 2, 10' rule when prescribing vision correction—assuming a working distance of about one foot for cell phones, two feet for desktop computers, and ten feet for television viewing. The new study is one of the first to examine the working distances at which patients use smart phones or other handheld devices.

The new study suggests that patients tend to use handheld devices at closer distances compared to printed materials, which could have important implications for vision correction. Smart phones "may present a variety of visual demands that are significantly different in terms of working distances, gaze angle, and text sizes," Rosenfield and coauthors write.

Further research is needed to investigate eye's response to prolonged smart phone use. In the meantime, the researchers believe that optometrists should ask patients in detail about how they use their handheld devices, and possibly perform further tests at those closer distances. Especially for older patients, changes in lens design may be needed to meet the visual demands of smart phones and other new technology.