TOKYO -- With each student in Japan receiving a tablet, digital devices nowadays are indispensable. At the same time, children's eyesight continues to decline. But are digital devices worse for the eyes than traditional books or comics?

When considering eye strain, the key factor, it appears, is the distance between the eyes and the object being looked at.

A 2015 study led by Naomi Nohara of Heisei College of Health Sciences, targeting 67 students aged 19 to 31, found that the average viewing distance when reading a book was 33.7 centimeters. In contrast, when composing emails on a smartphone, the average distance was 27.7 cm.

Ophthalmologist Takashi Fujikado, a specially appointed professor at Osaka University, commented, "When viewing an object at a distance of 20 cm, compared to 30 cm, around 1.7 times more focusing power is required. This increases the strain on the eyes, and we can say that digital devices, which are often viewed at closer distances than paper, are more likely to cause myopia."

In recent years, Japan has seen an increase, particularly among younger generations, of an eye condition known as "acute convergent strabismus," where one eye turns inward, causing double vision. The cause is unclear, but according to Fujikado, prolonged close-range viewing of digital devices could be one contributing factor.

The Japanese Association for Strabismus and Amblyopia and the Japanese Association of Pediatric Ophthalmology analyzed 194 patients between the ages of 5 and 35 with acute convergent strabismus, and found that those aged 16 formed the biggest group, at 16 patients, followed by 14 patients aged 13, and 13 patients aged 14 -- showing a high prevalence among junior and senior high school students.

Among these 194 patients reported from medical institutions nationwide, 156 who had been using digital devices for long periods had their screen time reduced for three months to see if their symptoms improved. For those up to elementary school age, usage was set at under one hour per day, while for those of junior high school age or older usage was limited to under two hours a day. Ten of them were cured of their strabismus, and 58 saw their symptoms improve. However, 88 of the patients, or nearly 60%, saw their symptoms remain unchanged or even worsen.

Fujikado explained, "In younger people, once the angle of eye misalignment becomes large, it becomes difficult to correct it own their own."

So what should children do to protect the health of their eyes? Fujikado recommends playing outdoors for two hours a day, keeping digital devices at least 30 centimeters away from the eyes and looking away from screens for two to three minutes every half an hour.

(Japanese original by Nanako Hori, Lifestyle, Science & Environment News Department)