Astigmatic correction

Very frequently associated with nearsightedness is an oval cross section to the corneal surface, called astigmatism. Imagine looking through a lens that was distorted to provide more focus vertically than horizontally. This is the case with astigmatism, (though it can be along any axis, it does not have to be vertically) with your cornea functioning as the lens.

Normal cornea	Reflection of circular ring onto cornea
Astigmatic cornea	Reflection of circular ring onto astigmatic cornea

Explanation:

Think of the normal cornea as being round, or spherical, with both horizontal and vertical curvature the same (two arrow diagram above). Think of astigmatism as if the cornea was pinched along the single arrow, or A. The circular ring reflection hits a plane of the cornea at the flatter curvature B before it does the steeper curvature at A that is further down - closer to the corneal base before reflecting onto the cornea there (further from the surface before reflected surface reached because it is pinched, or steeper vertically and the ring reflection hits a point further down). This creates an OVAL reflection - and visually a "streak effect". (The reflection has a smaller chord diameter along A (further away) and larger at B (closer)). This cornea would be described as having + cylinder at 90 degrees, or - cylinder at 180 degrees.

Every oval has a short and long diameter 90 degrees away from each other. The axis of the short diameter, known as the treatment axis of the positive astigmatism could be vertical, or any other direction over the 360 degree surface. A prescription written may look like this: -4.00 -2.00 x 90.

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