Refractive Errors and Focussing Problems

To see clearly the cornea and the lens must work together to focus light onto the back of the eye. The cornea, the front window of the eye, provides most of the eye’s focusing power. The lens, located behind the pupil, provides additional focusing power by changing shape to fine-focus images. The retina, in the back of the eye, captures the focused light images and sends them to the brain where they are interpreted into sight.

For light images to be focused on the retina, the eye must be correctly shaped. If an eye is too short or too long, the cornea and the lens are not able to focus light clearly on the back of the eye. A cornea with uneven curvature also prevents clear focusing.

Due to a variety of factors, many people have eyes that are not structurally balanced to provide good natural focusing. These structural imperfections result in different focusing problems: nearsightedness, farsightedness and astigmatism. Presbyopia is a focussing change that is age related.

Myopia - Nearsightedness

Nearsightedness or myopia, is the most common focusing problem affecting over 30% of the population. Nearsighted people can see things that are close, but they cannot see clearly in the distance. Nearsightedness usually results from an eye that is too long in relation to its focusing parts. Because of this extra length, the lenses cannot focus the light rays from distant objects on the retina. Instead, the images focus in the centre cavity of the eye and then diverge into a blur by the time they reach the retina. People with nearsightedness will almost always have the ability to read without glasses even when they grow older and presbyopia affects them. This is because their eye is already structurally suited to focus on objects that are up close.

Hyperopia - Farsightedness

Farsightedness or hyperopia, affects about 10% of people. Farsighted people may see things in the distance, wiath extra focusing effort, but they cannot see clearly up close, unless they are young and use additional effort. Farsightedness is a focusing problem usually resulting from a eye that is too short. Because of the reduced length, the muscles adjusting the lens may fatigue, or the lens may not be able to respond enough to focus the light rays. The retina thus receives a blurred image of light. The noticeable effects of farsightedness increase with age as the inner lens loses its ability to vary its focus to compensate for the problem. People over 50 years of age with farsightedness will always need a set of glasses to see in the distance and a stronger set of glasses for reading.

Astigmatism - Distorted Vision

Astigmatism is a refractive abnormality where light is not focused to a point on the retina and results in the general inability of the eye to clearly focus images from any distance because of unequal curvatures of the cornea. This is often caused by the cornea or front window of the eye being more elliptical (shaped more like an egg) rather than spherical (shaped like an orange).

Noone has a perfectly shaped cornea, but the distortion of most eyes is so slight that it does not significantly affect vision. However, more than a small degree of astigmatism can noticeably blur vision. Astigmatism can occur alone or along with nearsightedness or farsightedness.

Presbyopia is a universal focusing problem developing in most people around 45-50 years of age. When eyes are young, the gel inside the internal natural lens is soft so it can change shape to allow people to focus on things at various distances. But as the eye ages, the gel hardens and the lens enlarges and loses its ability to change shape to focus on near objects.

People with natural 20/20 distance vision will need reading glasses for close up focussing at age 45-50 . The same situation occurs after cataract surgery and lens implantation.