Corrective lenses modify the focal length of the eye to alleviate the effects of nearsightedness (myopia), farsightedness (hyperopia) or astigmatism.
As people age, the eye's crystalline lens loses elasticity, resulting in presbyopia,
which limits their ability to change focus.

The power of a lens is generally measured in diopters. Over-the-counter reading glasses are typically rated at +1.00 to +4.00 diopters. Glasses correcting for myopia
will have negative diopter strengths. Lenses made to conform to the prescription of an
ophthalmologist or optometrist are called prescription lenses and are used to make prescription glasses.