Asthenopia (aesthenopia) or eye strain is an ophthalmological condition that manifests itself through nonspecific symptoms such as fatigue, red eyes, eye strain, pain in or around the eyes, blurred vision, headache and occasional double vision. Symptoms often occur after reading, computer work, or other activities that involve tedious visual tasks.
When concentrating on a visually intense task, such as continuously focusing on a book or computer monitor, the inner eye muscles may tighten, which can cause the eyes to get irritated, dry, and uncomfortable. Giving the eyes a chance to focus on a distant object at least once an hour usually alleviates the problem. Small font sizes exacerbate the strain if they cause unconscious squinting or straining to focus.
On a computer, a CRT with a low refresh rate (less than 70 Hz) can cause similar problems because of the flickering image. Aging CRTs also often go slightly out of focus, and this can also cause eye strain. LCDs do not go out of focus and are less susceptable to visible flicker. Higher refresh rates and larger font sizes are worthwhile when addressing eyestrain problems.