About The Eye

Anatomy :: Of The Eye

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How To Read Eyeglass Prescriptions

At first glance, your prescription may be confusing with some unfamiliar abbreviations and numbers. But with the information listed here, you’ll be seeing a lot clearer.

OD represents your right eye. OS represents your left eye. OU refers to both eyes.

Sphere (SPH): This number represents strength of nearsightedness or farsightedness.

Cylinder (CYL): This number represents the amount of curvature/astigmatism (blur due to the shape of the cornea) in the eye.

Axis: This number represents the direction of the curvature of the eye surface. This number ranges from 1 to 180.

Add: This number represents ‘added’ power needed to focus in on close range activities--like reading, and is used for bifocal/trifocal prescriptions.

Prism: Very few prescriptions indicate prism, but this is prescribed to correct patients who suffer from eye muscle and/or focusing imbalances. Remember, your glasses prescription does not entitle you to contact lenses and is strictly for the purchase of eyeglasses. If you’re seeking contact lenses, you will have to set up a separate examination with your eye doctor.

Refractive Errors and Refraction: How the Eye Sees

Your eyes are windows to the world. We see when light reflects off objects and enters our eyes, passing through the clear outer cornea, the lens, and then creating an image on the retina which is transmitted to the brain. Refractive errors cause this image to be distorted or blurred. The most common types of refractive errors are nearsightedness, farsightedness, and astigmatism.

After an extensive eye examination, common refractive errors can be corrected with prescription eyeglasses or contact lenses. You can also treat refractive errors with LASIK, PRK, Visian ICL, and other correction surgeries. Eyesight is precious and should never be taken for granted. That’s why it’s important to schedule a routine comprehensive eye exam with your eye doctor.

The Eye Chart and 20/20 Vision

There are a number of different eye charts which can determine how well you see from a distance. The classic example is the Snellen eye chart. The Snellen eye chart shows 11 rows of capital letters which get progressively smaller as you read from top to bottom. The 20/20 line is usually the third line from the bottom. 20/20 vision is considered “normal,” and means that you are able to read a letter from 20 feet away that most people should be able to read at 20 feet away. Eye charts in offices are calibrated for different test distances, so that the rooms do not have to be 20 feet long.