Myopia or short-sightedness is a condition of the eye where the light that comes in does not directly focus on the retina but in front of it, causing the image that one sees when looking at a distant object to be out of focus, but in focus when looking at a close object. Myopia is corrected with negatively powered lenses. It may also be corrected by refractive surgery, though there are cases of associated side effects.
Astigmatism is a vision condition that causes blurred vision due either to the irregular shape of the cornea, the clear front cover of the eye, or sometimes the curvature of the lens inside the eye. An irregular shaped cornea or lens prevents light from focusing properly on the retina, the light sensitive surface at the back of the eye.
Hyperopia or longsightedness is a condition in which the optical components of the eye are not strong enough, and so light is not focussed onto the retina. This results in blurred vision that is usually worse at shorter distances.
A little hyperopia is not a problem because the lens compensates easily but if there is a significant amount of hyperopia, the effort of focusing (called accomodation) can lead to symptoms.
Hyperopic people may get tired eyes or headaches after a lot of visual work, even if the vision is clear. Reading is more difficult and school work can be affected. Because a hyperopic person often can see well in the distance, a letter chart test alone may miss hyperopia.