Many people require spectacles, contact lenses, or refractive surgery for clear vision. Whenever light comes into the eye, it must focus perfectly onto the neurosensory back of the eye, the retina.
Refractive error is a mistake in this process, where the eye either does not bend light properly or is too long or too short for light rays to perfectly intersect the retina.
The magnitude of the refractive error, measured between what an eye is doing and what perfect focus would be, is found by our eye doctor during a comprehensive eye examination. Refractive error comes in three main varieties, explained below.
Nearsighted and Myopiya
Nearsightedness or myopia is when the eye is either too long or too strong in how it bends light, in both cases resulting in light rays being focused in front of the retina rather than perfectly on it.
If someone is nearsighted and uncorrected, they will find that there is a distance close to them that renders clear vision but objects further away than this distance will be blurry.
Spectacles and contact lenses remedy this by providing negative power to diverge light before it enters the eye, thus allowing the eye to focus it on the retina. Refractive surgery works here by flattening the clear cornea at the front of the eye, making the eye overall less powerful in order to correct the myopia.
The amount of myopia may increase quickly during mid-late childhood and adolescence, resulting in a very high prescription and increased risk of eye issues in adulthood like retinal degenerations.
Our optometrist may therefore prescribe various methods to slow the rate of myopic progression if a child is advancing quickly, thus lowering the maximum prescription that they acquire and reducing the risk of further adverse events later in life.
Farsighted and Hyperopia
Farsightedness or hyperopia contrasts with nearsightedness in that the eye is either too weak at bending light or too short, resulting in light rays coming to a focus at a point behind the retina.
Patients below the age of 40 years with mild hyperopia will usually find that they can see well at distance and up close, but their eyes may get tired and strained if looking at a nearby object for a long time and they may have fluctuations in vision. This is because the eyes have a natural ability to increase focusing power for near that degrades as we grow older.
Glasses and contact lenses provide help to the eye to help it focus light onto the retina, relieving eye strain and clarifying vision. Refractive surgery in this case steepens the cornea, allowing the eye to have stronger refractive power.
Hyperopia is expected in young children but does not require correction unless the prescription is large, asymmetric between the eyes, or other factors are present.
Astigmatism is when the eye has a different magnitude of refractive error in different orientations, due to the cornea at the front or lens within the eye being more football shaped rather than spherical.
As an example, this might mean that a horizontal line is more clear to someone who is uncorrected than a vertical line, or vice versa, depending on the exact prescription. Astigmatism can be of any magnitude and any direction and is specified in the second and third numbers of a prescription, while the magnitude of myopia or hyperopia is told with the first number.
Astigmatism is corrected with neutralizing power at a specific orientation to correct the difference in prescription across the eye. This means that spectacles or contact lenses must sit in a precise direction to correct the astigmatism, and refractive surgery must appropriately steepen or flatten a specific orientation on the cornea.