Hyperopia, also known as far-sightedness, long-sightedness or hypermetropia, is a defect of vision caused by an imperfection in the eye (often when the eyeball is too short or when the lens cannot become round enough), causing difficulty focusing on near objects, and in extreme cases causing a sufferer to be unable to focus on objects at any distance. As an object moves toward the eye, the eye must increase its power to keep the image in focus on the retina. If the power of the cornea and lens is insufficient, as in hyperopia, the image will appear blurred.
People with hyperopia can experience blurred vision, asthenopia, accommodative dysfunction, binocular dysfunction, amblyopia, and strabismus.[1]
Hyperopia is often confused with presbyopia,[2][3] another condition that frequently causes blurry near vision.[4] Presbyopes who report good far vision typically experience blurry near vision because of a reduced accommodative amplitude brought about by natural aging changes with the crystalline lens.[4] It is also sometimes referred to as farsightedness, since in otherwise normally-sighted persons it makes it more difficult to focus on near objects than on far objects.[5]
[source http://en.wikipedia.org/wiki/Long_sightedness]
Astigmatism is an optical defect in which vision is blurred due to the inability of the optics of the eye to focus a point object into a sharp focused image on the retina. This may be due to an irregular or toric curvature of the cornea or lens. There are two types of astigmatism, regular and irregular. Irregular astigmatism is often caused by a corneal scar or scattering in the crystalline lens and cannot be corrected by standard spectacle lenses. Irregular astigmatism of the cornea can be corrected by contact lenses. Regular astigmatism arising from either the cornea or crystalline lens can be corrected by a toric lens. A toric surface resembles the surface of an American football or a doughnut where there are two regular radii, one smaller than another. This optical shape gives rise to regular astigmatism in the eye. [1] The first spectacle lenses that corrected astigmatism were made in Philadelphia in 1841.
The refractive error of the astigmatic eye stems from a difference in degree of curvature refraction of the two different meridians (i.e., the eye has different focal points in different planes.) For example, the image may be clearly focused on the retina in the horizontal (sagittal) plane, but not in front of the retina in the vertical (tangential) plane. Astigmatism causes difficulties in seeing fine detail, and in some cases vertical lines (e.g., walls) may appear to the patient to be tilted. The astigmatic optics of the human eye can often be corrected by spectacles, hard contact lenses or contact lenses that have a compensating optic, cylindrical lens (i.e. a lens that has different radii of curvature in different planes), contact lenses, or refractive surgery.
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