The prescription your eye doctor gives you for contacts is a little different from the prescription he gives you for glasses. This is because of the different way the two types of lenses fit on your face. It helps to understand how glasses and contacts each help correct your vision.
Glasses correct many types of vision problems, such as nearsightedness, farsightedness, astigmatism, and age-related changes in reading vision. Each lens is made to address the specific problems of each eye and then fit into a frame.
Contact lenses also correct many types of vision problems and are made specifically for each eye. They are fitted to sit directly on the eyeball.
The most common prescriptions include numbers for myopia (nearsightedness) or hyperopia (farsightedness). These corrections are measured in diopters, with myopia being treated with a negative number and hyperopia a positive one. For example, someone’s left eye could have a prescription of -3.5 diopters to correct nearsightedness while the right eye could be -2.5 diopters. Some people also have astigmatism, usually caused by an irregularly shaped cornea. This would be additional information in the prescription.
Eyeglasses place the correcting lens a short distance from the eye itself. Contact lenses, on the other hand, are placed directly on the eye. Because contact lenses are closer to the eye, they do not have to be quite as strong as eyeglass lenses to correct the vision. Therefore, contact lens prescriptions are usually a little less than the prescription for glasses. They also indicate the size, curve, brand, and type of lens. What’s more, someone with a very mild astigmatism might have it corrected in the glasses but not need a correction in the contact lenses.