THURSDAY, July 21 (HealthDay News) -- People reading text messages or browsing the Internet on their smartphones tend to hold the devices closer than they would a book or newspaper, forcing their eyes to work harder than usual, new research shows.

This closer distance -- plus the often tiny font sizes on smartphones -- could put added strain on people who already wear glasses or contact lenses, according to the study, which appears in the July issue of Optometry and Vision Science.

"The fact that people are holding the devices at close distances means that the eyes have to work that much harder to focus on the print and to have their eyes pointed in right direction," said study co-author Dr. Mark Rosenfield, a professor at the SUNY State College of Optometry in New York City. "The fact that the eyes are having to work harder means that people may get symptoms such as headaches and eye strain."

Texting and browsing the Web on smartphones can also result in dry eye, discomfort and blurred vision after prolonged use, the study authors point out. Previous studies have also found that up to 90 percent of people who use computers experience eye problems.

Rosenfield got the idea for the study while commuting to work on the train and noticing that people using smartphones seemed to be holding them very close to their eyes.

Given that more and more adults and children are using smartphones to write and receive messages or look up restaurant reviews, it made sense to measure exactly how close people were holding their phones.

The experiments were relatively simple ones. In the first, about 130 volunteers with an average age of 23.2 years were asked to hold their smartphone while reading an actual text message.

In a different experiment, 100 participants, whose average age 24.9, were next asked to hold their smartphone when reading a web page.

The researchers then measured the distance between the device and the eyes as well as the font size.

When reading printed text in newspapers, books and magazines, the average working distance is close to 16 inches from the eyes, but the study volunteers writing or sending text messages held their phones, on average, only about 14 inches away. In some people, it was as close as 7 inches, Rosenfield said.

When viewing a web page, the average working distance was 12.6 inches.

The font on text messages tended to be slightly larger (about 10 percent, on average) than newspaper print, but web-page font was only 80 percent the size of newspaper print and, in some cases, as small as 30 percent, Rosenfield said.

The findings hold messages for doctors and smartphone-users alike.

Given the ubiquitousness of these handheld devices, eye doctors might consider testing people's vision at closer distances and prescribing glasses for closer distances.

But there's a simple way for smartphone addicts to minimize eye strain: Increase the font size on your device, advised Dr. Scott MacRae, a professor of ophthalmology and of visual science at the University of Rochester Medical Center and an eye surgeon.

This is especially important for sustained reading, like reading a book on Kindle, he noted.

Font size on an e-book reader is usually pretty easy to do. For other handheld devices," MacRae said, "the problem is to figure out how to do it."

If you're a regular computer user, try using Verdana 12-point font, the only font designed specifically for computers, MacRae said.

The authors are now also assessing Kindles and IPads, but those results haven't been published.

More information

The American Academy of Ophthalmology has more on how to keep your eyes healthy.