Two main eye structures work together to focus images on the retina: the lens and the cornea. Sometimes, problems with the eye interfere with this focusing mechanism. The muscles then need to work harder, which can cause eye strain. Underlying conditions that can cause this to happen include the following:
Astigmatism – This refers to when the cornea is an irregular shape, causing objects to appear blurry. A person with this condition may then squeeze the eyes in order to focus on what they are trying to see, which can contribute to the onset of a headache.
Long-sightedness – The clinical term for long-sightedness is hyperopia. Here, an image is focused somewhere behind the eye instead of the retina. Nearby objects then appear blurry, meaning a person tends to use his muscles excessively in order to focus on them, which, again, can cause headaches.
Presbyopia – Here, the eye lens hardens as a result of aging, causing difficulty focusing, sore eyes and headaches.
These types of eye conditions can usually be corrected with the use of the right power of glasses.
Other causes of eye strain include any activity that requires intense use of the eye for long periods or in difficult conditions such as reading, writing, driving and straining to see in dim light.