Refraction is the bending of light as it passes through one object to another. Vision occurs when light rays are bent (refracted) as they pass through the cornea and the lens. The light is then focused on the retina. The retina converts the light-rays into messages that are sent through the optic nerve to the brain. The brain interprets these messages into the images we see.

You sit in a chair that has a special device called a phoroptor - looks like a big set of binoculars. You look through the device and focus on a chart with letters typically 20 feet away. The device contains lenses of different powers that can be rotated one at at time in front of each eye to determine which lens provides the best vision. The technician performing the test will ask if the letters on the chart appear more or less clear with different lens combinations.

A value of 20/20 is normal vision, meaning you are able to read 3/8-inch letters at a distance of 20 feet. If your uncorrected vision (without glasses or contact lenses) is normal, then the refractive error is zero (plano) - your vision should be 20/20.

You have a refractive error if you need a combination of lenses to see 20/20. Glasses or contact lenses should give you good vision. If your final vision is less than 20/20, even with lenses, then there is probably another, non-optical problem with your eye. The vision level you achieve during the refraction test is called the best-corrected visual acuity (BCVA).

The most common types of refractive errors are myopia, hyperopia, presbyopia, and astigmatism.

• Myopia (nearsightedness) is a condition where objects up close appear clearly, while objects far away appear blurry. With myopia, light comes to focus in front of the retina instead of on the retina.

• Hyperopia (farsightedness) is a common type of refractive error where distant objects may be seen more clearly than objects that are near. However, people experience hyperopia differently. Some people may not notice any problems with their vision, especially when they are young. For people with significant hyperopia, vision can be blurry for objects at any distance, near or far.

• Astigmatism is a condition in which the eye does not focus light evenly onto the retina, the light-sensitive tissue at the back of the eye. This can cause images to appear blurry and stretched out.

• Presbyopia is an age-related condition in which the ability to focus up close becomes more difficult. As the eye ages, the lens can no longer change shape enough to allow the eye to focus close objects clearly.

Other conditions under which the test may be performed:

Corneal ulcers and infections

Macular degeneration

Retinal detachment

Retinal vessel occlusion

Retinitis pigmentosa

YES, no IFs, ANDs or BUTs.

You should have a comprehensive eye examination every 3 - 5 years even if you have no vision problems. If your vision becomes blurry, worsens, or if there are other noticeable changes, schedule an eye examination immediately. After age 40 (or for people with a family history of glaucoma) eye examinations should be scheduled at least once a year to test for glaucoma. Anyone with diabetes should also have an eye exam at least once a year. People with a refractive error should have an eye examination every 1 -2 years, or whenever their vision changes.