Eye strain, also known as asthenopia, is an eye condition that manifests through non-specific symptoms such as fatigue, pain in or around the eyes, blurred vision, headache, and occasional double vision. Symptoms often occur after long-term use of computers, digital devices, reading, driving long distances or other activities that involve extended visual tasks.
When concentrating on a visually intense task, such as continuously focusing on a book or computer monitor, the ciliary muscles and the extraocular muscles are strained. This causes discomfort, soreness or pain on the eyeballs. Closing the eyes for ten minutes and relaxing the muscles of the face and neck at least once an hour usually alleviates the problem. It is more important to ensure seven to eight hours of uninterrupted sleep every night to allow the tissues to heal.
A CRT (cathode ray tube) computer monitor with a low refresh rate (<70 Hz) or a CRT television can cause similar problems because the image has a visible flicker. Even if this flicker is imperceptible, it can still contribute to eye strain and fatigue. Aging CRTs also often go slightly out of focus, and this can cause eye strain. Old tube-style monitors can be replaced with a flat-panel LCD (liquid crystal display) like those on laptop computers. LCD screens are easier on the eyes and usually have an anti-reflective surface.
A page or photograph with the same image twice, but slightly displaced (from a printing mishap, a camera moving during the shot, etc.) can cause eye strain due to the brain misinterpreting the image fault as diplopia and trying in vain to adjust the sideways movements of the two eyeballs to fuse the two images into one.
Eye strain can also happen when viewing a blurred image (including images deliberately partly blurred for censorship), due to the ciliary muscle tightening trying in vain to focus the blurring out. Subtle blurring is also induced by the poor refractive properties of plastic and polycarbonate lenses. Glass lenses are known to offer better visual acuity.