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Refractive Error

REFRACTIVE ERROR

Refractive error is the term given to the blur caused by light entering the eyeball that does not focus perfectly onto the retina (the inner lining of the back of the eye). The refractive error is what optometrists or ophthalmologists measure during an eye examination to determine the prescription for spectacles or contact lenses.

Light entering the eyeball is refracted or bent mainly by the cornea (clear covering of the eye) and the crystalline lens (natural lens inside the eyeball). The ability of the eyeball to focus light on the retina determines our level of refractive error.

Emmetropia is the term used when light is perfectly focused by the cornea and crystalline lens on the retina without the use of spectacles or contact lenses. Obviously everyone wishes they were an emmetrope.

Hyperopia or far-sightedness occurs when light is focused behind the retina. This is caused by the eyeball being too short or the refractive structures in the eye being too weak. Typically hyperopic patients have difficulty seeing near objects without spectacles or contact lenses.

Myopia or near-sightedness occurs when light is focused in front of the retina. This is caused by the eyeball being too long or the refractive structures in the eye are too strong. Typically myopic patients have difficulty seeing distant objects without spectacles or contact lenses.

Astigmatism is a term given to the situation where light is focused at two points. This is caused by the cornea or the crystalline lens becoming football-shaped as apposed to being perfectly round. Depending on the degree of astigmatism objects at near and far can be blurred.

Presbyopia is the term used when there is difficulty focusing on near objects. Everyone will experience this situation in their lifetime. Presbyopia typically begins in your late thirties or early forties. This phenomenon occurs due to the weakening of the muscles in the eye that control the focusing mechanism or the increased rigidity of the crystalline lens or a combination of both.

Frequently I am asked: what does 20/20 vision mean? It is difficult to explain so here is my best shot. The degree of one’s vision is called visual acuity. Visual acuity is measured by a Snellen chart. The Snellen chart has a series of lines of letters in descending size order. We consider 20/20 to be “perfect” vision. If someone had 20/40 vision that person’s vision would be twice as bad as a person with 20/20. In other words if you showed that 20/40 patient a letter that was visualized perfectly (at a 20/40 sized letter on the Snellen eye chart) he/she could back up as far as 20 feet from the Snellen chart until they would not be able to identify that letter. On the other hand the patient with 20/20 vision could back up to a maximum of 40 feet until he/she could not identify that same letter. The lower the number after the first “20” the better the vision.




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