Vision Facts...
-What is Emmetropia, Farsighted, Nearsightedness and Astigmatism?
If the eye muscles are not working and the image of the world that the eye sees at 20 feet or farther away is clear without a refractive correction, this is called EMMETROPIA. The average person's eyes reach adult size at approximately twelve years of age. A relaxed emmetropic eye that is seeing clearly in the distance must then focus a normal amount to see up close. This near focusing without any refractive correction is usually possible up until 38 to 40 years of age for an emmetrope.
When an eye must focus to see far away, this is called FARSIGHTED or hyperopic. Most human eyes at birth are, in fact slightly farsighted, and need to focus a small amount to see clearly in the distance. This allows the normal growth of the farsighted eye until about age twelve years to reduce the farsightedness toward emmetropia, and allows for farsighted children to have a clear image on the retina at all times. A farsighted eye focuses to see clearly in the distance, and then must focus even more than an emmetropic eye to see up close. This makes a farsighted eye need help focusing at near with the best distance correction sooner than the 38 to 40 year old emmetropic eye, and will usually progress to a point where the farsighted eye will need a refractive correction to see clearly in the distance. Farsighted corrections are usually not necessary for children unless the power is so high that it becomes difficult for the child to keep the near image clear on the retina clear.
A NEARSIGHTED or myopic eye relaxes as much as it can, trying to see in the distance, but cannot see the distance clearly. If an eye is nearsighted at birth, the image on the retina is always blurry when the child looks far away. Studies have shown that a blurry image on the retina causes the white part of the eye, or sclera, to soften, and combined with normal eye growth, results in more total eye enlargement than an emmetropic eye, and thus makes a nearsighted eye become even MORE nearsighted. This can commonly progress up to around twenty years of age unless a clear image is maintained on the retina at all times. Most children and adults who are nearsighted can see clearly at near without a refractive correction. A nearsighted eye will typically need help focusing at near when wearing the best distance correction later than the 38 to 40 year old emmetropic eye. Many nearsighted adults will simply remove their distance correction to see clearer at near instead of wearing a near correction.
ASTIGMATISM describes the condition where light entering the eye focuses over a range (Sturm's Conoid) rather than a point focus. This is the result of the combination of the shape of the surface of the eye (cornea) and the crystalline lens inside the eye. Astigmatism can be corrected by spectacle lenses, contact lenses or LASIK treatment of the cornea.