Why Digital Devices Cause More Eye Strain
In addition to the long hours people spend on their computers and portable electronic devices these days, the nature of the digital screens themselves contribute to increased potential for eye strain, compared with reading the same information printed on paper.
This is because characters on computer screens are made up of tiny dots or squares of light called pixels (literally, "picture elements"), and lack the uniform density and sharply defined borders of text characters printed with ink on paper.
Even on high-definition displays (such as Apple's Retina Display), text characters formed with pixels can be less distinct than those formed on a printed page. This is especially true if you tend to hold your smart phone or other device closer to your eyes than the recommended viewing distance. (The resulting reduced image quality from holding your device too close is called pixelation, which can be more apparent with some font styles than others.)
Also, even the best-quality images formed on computer screens are more difficult for the eye to stay focused on, compared with print on paper. Constant refocusing causes fatigue of the focusing muscle inside the eye, which is a major component of digital eye strain.
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