Corrective lenses are glass or plastic lenses worn over, on, or in the eye to help the eye focus so a person can see clearly. There are three main types of corrective lenses: glasses, contacts, and intraocular lenses. An eye care professional such as an ophthalmologist or optometrist can determine whether or not a person needs corrective lenses with a routine eye exam, and the exam can also be used to arrive at a prescription for a specific pair of lenses.
Eyeglasses consist of a frame with glass or plastic lenses that sits on the bridge of the nose and is supported by two legs, called temples, that rest on the ears. Glasses come in several varieties, primarily:
Eyeglass lenses are often clear, but can be tinted for aesthetic reasons or for use as sunglasses. Some eyeglasses change opacity when exposed to sunlight, transitioning into sunglasses when the wearer steps outside into bright light.
Other types of eyeglasses include:
All types of glasses need to be cleaned regularly with a microfiber cloth and a cleaning solution or a mild mix of dish soap and lukewarm water. This helps ensure that the wearer can see properly and can also keep the lenses from getting scratched by debris. Glasses should be stored in a hard case when not in use to keep them from getting dirty and to protect them from damage.
Most eyeglass lenses require a prescription in the United States, but there are mildly corrective non-prescription glasses available for reading, or less commonly, for nearsightedness. Though these glasses are usually cheaper than prescription glasses, they are not tailored to the person wearing them, and in some cases can exacerbate eye problems.
Contact lenses are small, round lenses that are placed directly on the eye's surface. Most modern contact lenses are made of plastic or silicone, though older contact lenses were made of glass. Most contacts are almost invisible when worn, except for contacts that are intentionally colored. Some contacts are intentionally tinted or colored so that the wearer can see them more easily if they fall out, or for cosmetic reasons.
Contacts come in:
All types of contacts except for daily use contacts need to be cleaned and stored in a sterile environment when they are not being worn. Caring for contacts is particularly important, since they come into direct contact with the eye. Unclean contacts can encourage or cause problems with the eyes including eye infections, corneal abrasions, and in rare cases, blindness. Some contacts need to be cleaned differently than others, and there are a variety of contact cleansers, including enzymatic cleaners and ultraviolet cleaners, so wearers should always follow the instructions of their ophthalmologist.
In the US, all contacts are prescription-only, even non-focal ones and colored ones. Though some vendors sell contacts to those without a prescription, they are illegal, and should not be bought from.
Intraocular lenses are actually implanted into the eyeball, sometimes to replace a lens that has been removed or has become unusable due to cataracts or glaucoma, but also simply to correct vision. Most intraocular lenses are monofocal, meaning that wearer may have trouble focusing at distances other than the one that the lens is designed for. They are primarily focused for distance vision, though multifocal intraocular lenses do exist. Intraocular lenses can be placed in about 30 minutes by an experienced ophthalmologist in an outpatient procedure. Recovery time after getting intraocular lenses is usually about two to three weeks.
Since intraocular lenses are not removed, they do not require care once they are in the eye, but during the recovery period after the lens is implanted, the wearer cannot do heavy exercise or things that raise his or her blood pressure.
How Corrective Lenses Work
Corrective lenses work by bending light to move the focal point of the light coming into the eye closer or further away. In a normal eye, the natural lens of the eye bends this light to the right length for it to focus on the retina, the back of the eye, and the eye can focus without help, but some people's eyes bend the light so it focuses before the retina or at a point behind the retina, which makes their natural vision unfocused.
People with myopia, or nearsightedness, have an eye shape that causes the light to focus in front of their retina, so they need lenses that move their focal point further away to make it possible to see things in the distance. Farsighted or hyperopic people have eyes that focus the light at a point behind their retina so they need lenses that bring the focal point closer so that they can see objects close up. The picture below shows a cross-section of light focusing in a normal eye on the top, a farsighted eye in the middle, and a nearsighted eye on the bottom.
The depth of the curve of a corrective lens, the thickness of the lens, and the precise shape of the lens can all be used to change the focal point. Even small changes in the degree of the curve of a lens can make a significant impact to vision. Lenses for those with myopia are convex, shaped like a thin, tall football, while lenses for hyperopia are concave, with the middle part being thinner than the ends.
There are also special types of corrective lenses for those with astigmatism. While those with myopia, or nearsightedness, have longer or egg-shaped eyeballs and those with farsightedness have eyeballs that are shorter than normal, those with an astigmatism have irregularly shaped corneas — not eyeballs — which makes their vision blurred. People with astigmatism need a special type of corrective lens called a toric lens, which bends light more on one part than the other.
The strength of a corrective lens is measured in diopters, which is written in a plus or minus form in a lens prescription. So a lens with a 2.5 diopter measurement for a person who is nearsighted would be written as “-2.5” on his or her prescription or contact lens box. The lenses below are classified according to their diopter, and are used by ophthalmologists during an eye exam to help determine what level of prescription a person needs.
Most people can only distinguish between quarter-diopter changes, so most corrective lens prescriptions are written in quarter-diopter amounts (+1.00, +1.25, +1.50, etc.) In the US, diopter measurements are written in the negative for nearsightedness and in the positive for farsightedness, though this can be the opposite in other countries.
Pros and Cons of Corrective Lenses
Some studies suggest that in those with borderline myopia or hyperopia — particularly children — wearing corrective lenses may lead to dependence on the lenses and may cause their eyesight to worsen, but this has not been conclusively proven. For most people, wearing corrective lenses of an appropriate prescription will not cause their vision to worsen.
Wearing corrective lenses with an incorrect prescription can cause eye strain, headaches, and dizziness, and wearing unsuitable contact lenses, particularly those that do not fit the eye properly, can lead to serious eye problems, including scarring.
Despite this, corrective lenses are valuable for correcting vision, and can be a necessity to drive, do some types of work, read, or for a good quality of life.
Pros and Cons of Specific Types of Lenses
Alternatives to Corrective Lenses
The most basic alternative to corrective lenses is simply not using them and living with the impaired vision. Other alternatives for correcting vision include:
Video 1 — A tutorial on cleaning glasses.
Video 2 — A tutorial on cleaning contact lenses.