Refractive error
What is it?
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| The Allegretto Wavelight laser used by Dr McKellar at LaserVision |
Refractive error is present when light from a distant object does not come to a clear focus on the retina.
Types of refractive error
There are four types of refractive error:
- Myopia or shortsightedness
- Hyperopia or long sightedness
- Regular astigmatism
- Irregular astigmatism
Myopia
Myopia is the medical term for nearsightedness. A myopic eye may be considered as having too steep a cornea or too long an axial length so that light from a distant object is brought to a focus not on, but in front of the retina.
Hyperopia
The medical term for farsightedness. A hyperopic eye may be considered as having too flat a cornea or too short an axial length so that light entering the eye comes to a focus not on, but behind the retina.
Regular astigmatism
An eye with regular astigmatism has different optical power in two different planes. The simplest form to understand is where light from a vertical line comes to a focus in front of the retina whereas that from a horizontal line focuses behind the retina. The cause of astigmatism is usually an asymmetry of the corneal surface.
Irregular astigmatism
Irregular astigmatism, also known as higher order abberation, is non-symmetrical irregularity in the optical components of the eye, usually in the shape of the cornea.