To write your eyeglasses or contact lens prescription, your ophthalmologist must determine your exact refractive error. Your correction is expressed in diopters that measure how much a corrective lens must bend light to focus it on your retina to normalize your vision. A lens that can bend parallel light rays to a focal point of 1 meter is said to have a power of 1 diopter (1.00D). A 2-diopters lens can focus light rays at a point 0,5 meters away from itself.

If you are blessed with normal eyesight, your ophthalmologist will write in your chart that your "sphere" is 0.0D, or "plano" (pl). This means, of course, that you have no refractive error. Department store sunglasses without power are plano lenses.

If you have a myopic correction of -1 diopter, you are focused at 1 meter, or approximately the length of your arm. Without eyeglasses or contact lenses, objects farther away than your hand will be slightly blurred and will become progressively more indistinct the further in the distance you look. With this refractive error, and no other visual defects, you should be able to read most street signs in good light. If your refraction is -2 diopters, you are focused at 0,5 meters, and you can only see clearly halfway down your arm. With this refractive error, you dare not drive without eyeglasses or contact lenses. Nonetheless, you can see better in the distance than a person who is more nearsighted – someone, for example, whose correction is -3 diopters.

In case, that you are farsighted, your amount of correction is expressed in positive numbers (+1 diopter of hypermetropia, e.g.).

With myopia, a refractive or focusing error less than -5 diopters is considered mild "myopia". Myopia is the medical term for nearsightedness (shortsightedness) and is corrected with a "minus: lens. Five to 7 or 8 diopters is in the moderate range, and greater than 7 or 8 is called severe. About 90 percent of nearsighted people have a refractive error of less than -6 diopters. Only about one nearsighted person in ten has severe myopia. Nevertheless, even if you have only -4 diopters of nearsightedness, you may feel that your correction is severe because you are almost totally dependent on your eyeglasses or contact lenses. With astigmatism, less than 1 diopter is considered mild. One to 2 diopters is moderate. Two to 3 diopters is severe, and greater than 3 diopters is extreme. With farsightedness, greater than +5 diopters is called severe "hyperopia or hypermetropia", the medical term for farsightedness. This refractive error is corrected with a "plus" lens.