Glasses and contact lenses correct refractive errors by adding or subtracting focusing power to your cornea and lens. The power needed to focus images directly on your retina is measured in diopters. This measurement is also known as your eyeglass prescription.
If you have myopia, your cornea and lens have too much focusing power, bending light rays to meet at a point in front of the retina. Glasses and contacts compensate for this condition by subtracting power from the eye's natural focus and allowing light rays to focus further back on the retina. If you have myopia, your prescription will be negative, for example, -4.25 diopters.


Myopia correction

 

If you have hyperopia, glasses and contacts add focusing power, causing light rays to bend more as they enter the eye. This process moves the focal point back to the retina, allowing for clear vision. If you have hyperopia, your prescription will be positive, for example, +4.25 diopters.


Hyperopia correction

If you have astigmatism, the shape of the glass lens compensates for the uneven corneal curve and focuses the light rays to a single point on the retina.