Myopia – nearsightedness caused by an eyeball too long for the convexity of the lens.
	Correction is a concave lens to reduce convergence.
	Hyperopia – farsightedness caused by an eyeball too short for the convexity of the lens, or to weakened ciliary muscles.
	Correction is a convex lens to increase convergence.
	Presbyopia – “old eyes” – loss of elasticity impairs focusing at various distances. Correction is usually bifocals.
	Astigmatism – irregular curvature of a misshapen lens or cornea.
	Correction is a lens which counters the abnormal curvature.