What Happens If Contact Lenses Are Made Too Thick?

Contact lenses, like glasses, are corrective lenses for the eye, with the major difference being that contacts are worn directly on the cornea. The mechanics by which vision is corrected is the same, however, and in either case if the lenses are too thick, too thin or otherwise misfitted for the wearer, they will not correct eyesight as they are intended to. A more in-depth look at how the eye works, basic eyesight problems and how corrective lenses help fix those problems will explain why.

How the Eye Works

• Vision is a science of light and lenses very similar to a camera. Because the eye is a sphere, it has both convex (pointing out) and concave (pointing in) lenses. Light enters the eye through the cornea, the convex lens at the front of the eye. The light is refracted, or bent, and focused on the retina at the back of the eye in the same way that a camera lens focuses light onto the film plate. The image is projected upside down when it hits the retina, just as images appear upside down on the film of a camera. We see images as right side up because the retina is lined with nerve cells that change the light rays into signals that are processed by the brain as images right side up.

Common Eyesight Problems and Their Causes

• Eyesight problems have innumerable causes, but the four most common are myopia, hyperopia, presbyopia and astigmatism. Myopia, or nearsightedness, is the result of an eyeball being too long. Nearsighted people can see close-up, but their vision is blurry on objects in the distance. Hyperopia is farsightedness, caused by an eyeball being too short. In this case, people can see objects far away but not close. Presbyopia often occurs as the eyeball ages and loses the ability to shift focus from near to far, resulting in problems seeing both close and far. And finally, astigmatism is caused by a cornea that is irregularly shaped, which results in distorted vision.

  In each of the four basic eyesight problems, blurry or distorted vision is caused by light waves failing to focus on the correct point on the retina. When the eyeball is too short, light refracts to a focal point behind the retina, an elongated eyeball results in a focal point in front of the retina, an irregular cornea results in light converging at different points and when the lenses no longer change shape to shift focus from near to far as in presbyopia, light no longer converges at one place on the retina and objects at various distances will be blurry.

What Are Contact Lenses?

• Contact lenses are discs worn on the eyeball over the iris and pupil to correct common vision problems. Lenses can be soft or hard, colored or clear, disposable or extended wear, gas permeable or silicone hydrogel. In the United States, contact lenses are classified as medical devices and cannot be purchased without a prescription from a licensed optometrist or ophthalmologist. Like glasses, contacts must be prescribed to fit each user, and to ensure a proper fit, many measurements will be taken. A keratometer will measure the curve of the cornea to make sure that contacts are angled correctly; otherwise, the cornea could be damaged. The tear film produced by the eye is measured to make sure the eye is moist enough for contacts, and a biomicroscope measures the health of the cornea and determines how much the contacts move with each blink. Measurements are made several times because contacts can tighten as they are worn and often need adjustments.

How Contact Lenses Fix Eyesight Problems

• Contact lenses are shaped according to the eyesight problem they are meant to correct. Lenses built to correct myopia are concave, or thinner at the center than the edges. This shape bends light away from the center of the lens and moves the focal point up to where it converges on the retina. The reverse is true of lenses built to correct hyperopia. These lenses are convex, or thicker at the center than the edges. This shape bends light toward the lens's center and pushes the focal back to the retina. Because presbyopia is a combination of both problems, lenses must allow for both near and far correction and so are built with both shapes in one lens. These are referred to as bifocal or multifocal lenses.

  In the case of astigmatism, toric lenses are specially made to accommodate each particular cornea, because the irregularity varies from person to person. Toric lenses will be thick and thin as needed to make the overall cornea smooth and even, and they are weighted at the bottom or built with thin edges top and bottom to ensure that they stay in place on the eye.

Incorrect Contact Lenses Specifications

• Whether the contact lenses are built to correct nearsightedness or farsightedness, presbyopia or astigmatism, the thickness of the lens is an indicator of the prescription strength. Every pair of corrective lenses is prescribed to adjust for the individual's eye strength, and will have a specific diopeter number, or, light-bending strength, accordingly. A stronger prescription has a higher diopeter number, which means that the lens must bend light more in order to make the light converge at the right place on the retina to provide clear vision, and the lenses are proportionally thick. If a lens is too thick, the light will not be bent at the proper angle to correct vision, and other problems can also result, such as headaches, eye strain and blurred vision.