Longman Dictionary of Contemporary English
Wikipedia
Eye strain also known as asthenopia is an eye condition that manifests itself through nonspecific symptoms such as fatigue, pain in or around the eyes, blurred vision, headache and occasional double vision. Symptoms often occur after reading, computer work, or other close activities that involve tedious visual tasks.
When concentrating on a visually intense task, such as continuously focusing on a book or computer monitor, the ciliary muscle tightens. This can cause the eyes to get irritated and uncomfortable. Giving the eyes a chance to focus on a distant object at least once an hour usually alleviates the problem.
A CRT computer monitor with a low refresh rate (<70 Hz) or a CRT television can cause similar problems because the image has a visible flicker. Aging CRTs also often go slightly out of focus, and this can cause eye strain. LCDs do not go out of focus but are also susceptible to flicker if the backlight for the LCD uses PWM for dimming. This causes the backlight to turn on and off for shorter intervals as the display becomes dimmer, creating noticeable flickering which causes eye fatigue.
A page or photograph with the same image twice slightly displaced (from a printing mishap, or a camera moving during the shot as in this image) can cause eye strain by 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. The word is from Greek "asthen-opia: ασθεν-ωπία".
Usage examples of "eye strain".
Like those lines and symbols in the optician's that you squinted at trying to make up your mind which ones were level, and which were thicker, and eventually you finished up with eye strain anyway so they sold you a new pair of lenses.
In order to obviate eye strain the walls were painted somberly and there were no light colors in evidence except for a nearly square patch of white at the farther end, the screen upon which the pictures were projected.