Astigmatism, Myopia & Hyperopia
Refractive Errors explained
Astigmatism, Myopia & Hyperopia happens when the refraction of light through the eye does not fall in a point focus exactly on the retina. This is called a refractive error. These refractive errors are very common as it is highly unlike to have Emmetropia (No Refractive Error)
Astigmatism
Astigmatism is the condition when incoming light is not refracted the same across all directions in the eye. The end result is that there is no sharp point focus on the retina. Astigmatism can be regular or it can be irregular.
In the case of regular astigmatism we can correct it with spectacles or with normal contact lenses. Regular astigmatism is usually symmetrical between the two eyes.
In the case of irregular astigmatism we can correct it with special contact lenses called sclerals.
Myopia
Near-sightedness or Myopia, is that condition where the focus point of parallel light rays is just in front of the retina. This is in reference to an unaccommodated eye. The near-sighted / myopic eye has too much plus power for the length. The miopic / near-sighted eye has a fixed far point. A person with a -1.00 D prescription can see an object clearly if it is 1 meter from the eye.
There are different types of myopia.
Axial-Myopia : The eyeball is too long for the normal power of the cornea and lens of the eye. The focal point of parallel light is in front of the retina. This eye is longer than the average eye length.
Curvature-Myopia : In this case the eye has a normal size or length but the curvature of the cornea is too steep.
Index-Myopia : This myopia is due to the change in the refractive index of the lens in the eye. This happens with diabetes and with cataracts.
Hyperopia
Hyperopia occurs when the refractive power of the eye is not sufficient to focus the image on the retina, but rather behind the retina. THe image falling onto the retina is blurred and out of focus. The only way to bring the image into focus is either by placing a lens in front of the eye or to accommodate the eye the focus the image. In most cases, hyperopia is due to the eye not being long enough. Each millimeter of axial length change is the same as 3 Diopter change. Another cause for Hyperopia is when the curve of the lens or cornea is not steep enough and too flat. Usually young people do not notice a problem because they are really good at focussing the image on the retina through accommodation.

Astigmatism

Myopia
