Contact lenses are worn directly on the cornea of the eye. Like eyeglasses, contact lenses help to correct refractive errors and perform this function by adding or subtracting focusing power to the eye's cornea and lens. This measurement reflects the amount of power necessary to focus images directly on to the retina. In addition, the eyeglass prescription may also contain the following measurements. This number measurement reflects the extent of the nearsightedness or farsightedness.
Convex lenses are thickest in the center, like a magnifying glass.
Cylindrical lenses curve more in one direction than in the other and are often used to correct astigmatism. Special measurements will need to be taken of the curvature of the eye. In addition, the physician will determine if the eyes are too dry for contact lenses, and if there are any corneal problems that may prevent a person from wearing contact lenses. Trial lenses are usually tested on the eyes for a period of time to ensure proper fit. Although, many eye care specialists will give you a copy if you request one.