In both conditions light does not focus properly on the retina. In myopia, the eyeball is elongated and light focuses in front of the retina. In hyperopia, the eyeball is shortened and light focuses at a point in back of the retina. Astigmatism is caused by an irregularity in the shape of the cornea that causes light to focus in different planes. Corrective lenses reduce the multiple images and refocus them to the retina.
Eyeglass frames can also play a role in eye safety.
Well-fitted frames should neither pinch the bridge of the nose nor lie too heavily behind the ears. Thicker lenses require heavier frames. The ideal color for tinted lenses is gray because it is the most neutral in the color spectrum, letting most colors appear in their natural hue. Green and brown are considered next best. Darkly tinted lenses should be limited to outdoor use because they can severely limit visibility indoors.