Vision

ASTIGMATISM
INTRODUCTION.
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 bedue 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 thecrystalline lens and cannot be corrected by standard spectacle lenses, but can be corrected by contact lenses. Regular astigmatism arising from either the cornea or crystalline lens can be corrected bya toric lens. A toric surface resembles a section of the surface of an American football or a doughnut where there are two regular radii, one smaller than another. This optical shape gives rise toregular astigmatism in the eye. 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 ofcurvature 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 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 radiiof curvature in different planes), or refractive surgery.
TYPES.
Based on axis of the principal meridians.
Blur from astigmatic lens at different distances.
* Regular astigmatism – principalmeridians are perpendicular
* With-the-rule astigmatism – the vertical meridian is steepest (an American football lying on its side).[2]
* Against-the-rule astigmatism – the horizontal...