The biggest knock on smartphones so far has been that all the apps, e-mails, viral videos and text messages drive us to distraction, if not off the road. But spending half the day staring into a 4-inch screen may also strain one’s eyesight, new research suggests. Surprisingly, the devices may not be to blame so much as how we hold them.

David Allamby, an eye surgeon and the founder of Focus Clinics in London, said that according to his research, there has been a 35 percent increase in the number of people with advancing myopia since smartphones were launched in 1997. Allamby coined the term “screen sightedness” to describe it.

Nearsightedness, or myopia, affects more than 30 percent of the U.S. population. It’s caused by a combination of hereditary factors and environment, says Dr. Shlomit Schaal, an eye surgeon and assistant professor of ophthalmology at the University of Louisville, Ky. There is no known way to prevent myopia, or even to slow it down. Glasses and contact lenses don’t affect its progression, Schaal says. The environmental factors that contribute include “close work,” or stress on the eye caused by reading or otherwise focusing on an object that is close.

Using a smartphone strains the eyes in much the same way reading a book or staring at a computer monitor does, with one exception: the distance between the eye and the object.

When a phone or other device is held close to one’s face, it forces the eye to work harder than usual to focus on text, says Dr. Mark Rosenfield, an optometrist who published research on the topic in the Journal of the American Academy of Optometry. The discomfort can eventually result in fatigue.

People tend to hold smartphones considerably closer to their faces than they would a book or newspaper, even as close as 7 or 8 inches, Rosenfield says. And since smartphones have such a small screen, the incidence of peering closely at them tends to be higher than for other devices.

Holding a smartphone farther away, but still using it the same amount, won’t necessarily prevent myopia, Schaal said. But holding the phone at least 16 inches from the face during use can be beneficial, Rosenfield says.

He also suggests taking breaks from using the phone. During those breaks, it is helpful to look into the distance, which relaxes the eye as it focuses on faraway detail instead of what is close.

Young children’s eyes may be spared early damage by limiting smartphone and tablet use, doctors say. Spending hours playing games or otherwise intently viewing a screen causes children’s eyes to exert effort for long periods. In the past, children focused on larger objects like blocks or toys, rather than such fine detail. They should be encouraged to engage in a variety of activities with different focusing targets of both near and faraway objects, Schaal says.

It’s not all bad news when it comes to your eyes and electronic devices, though. For people who already have myopia, tablets and other devices can actually help.

People with age-related macular degeneration have benefited from being able to view larger fonts and increased contrast on handheld devices like an iPad or Kindle, Schaal said.

“In the past, these patients might have had to use a magnifying lens or very strong glasses to read the material, but now they can enlarge the print and read it with a more normal prescription,” Rosenfield added.