Corrective lenses are used to correct refractive errors of the eye by modifying the effective focal length of the lens in order to alleviate the effects of conditions such as nearsightedness (myopia), farsightedness (hyperopia) or astigmatism. Another common condition in older patients is presbyopia which is caused by the eye's crystalline lens losing elasticity, progressively reducing the ability of the lens to accommodate (i.e. to focus on objects close to the eye).
The power of a lens is generally measured in diopters. Glasses correcting for myopia will have negative diopter strengths, and glasses correcting for hypermetropia will have positive diopter strengths. Glasses correcting for astigmatism require two different strengths placed at right angles in the same lens. Prescription lenses, made to conform to the prescription of an ophthalmologist or optometrist, are used to make prescription glasses, which are then verified correct using a professional lensmeter.
Eyeglasses can normally correct and compensate for four types of vision deficiencies:
- Myopia is a vision disorder that causes far objects to appear blurred but near objects are seen clearly. Individuals suffering from myopia are prescribed eyeglasses with concave lenses, which compensate for the refraction error by moving the image of the distant objects that cannot be seen clearly backward onto the retina.
- Correcting hyperopia is normally done with eyeglasses with convex lenses. With this disorder, the patients can see distant objects clearly but they have trouble with seeing objects that are close to them. Eyeglasses with convex lenses compensate for the refraction errors by moving the image of a distant object forward onto the retina.
- Astigmatism is typically corrected with a cylindrical lens. This disorder is caused by a non-uniform curvature in the refractive surfaces of the eye, which leads to an abnormality in focusing the light rays on the retina. As a result, a part of the light rays are focused on the retina and the other part is focused behind it or in front of it.
- Presbyopia is more frequent in people over 40 years old and it is corrected with convex lenses. These patients need reading or bifocal eyeglasses.
Scratch-resistant coatings can be applied to most plastic lenses giving them similar scratch resistance to glass.Hydrophobic coatings designed to ease cleaning are also available, as are anti-reflective coatings intended to reduce glare, improve night vision and make the wearer's eyes more visible.
Magnifying lenses or generic spectacles that are used to treat mild presbyopia and hyperopia can be bought off the shelf. Although such glasses are generally considered safe, an individual prescription usually results in better visual correction and fewer headaches & visual discomfort. There have also been many cases where people have delayed having a proper eye examinaton with an optometrist or, preferring to purchase off the shelf glasses, who have put their sight at risk from conditions such as AMD, Glaucoma and complications from Diabetes. It is important to stress off the shelf readers are not a replacement for regular eye examinations.
Reading glasses come in two main styles: full frames, in which the entire lens is made in the reading prescription, and half-eyes, the smaller "Ben Franklin" style glasses that sit lower down on the nose.
Full reading glasses are more suitable for people who only need them for close-up reading while half-eye reading glasses can be used to read at smaller or larger distances. The reading glasses are most of the time needed by people who have never worn glasses.
Bifocals, trifocals, and progressive lenses
As people age, their ability to focus is lessened and many decide to use multiple-focus lenses, which can be bifocal or even trifocal, to cover all the situations in which they use their sight. Traditional multifocal lenses have two or three distinct horizontal viewing areas, each requiring a conscious effort of refocusing. Some modern multifocal lenses, such as progressive lenses (known as "no-line bifocals"), give a smooth transition between these different focal points, unnoticeable by most wearers, while other glasses have lenses specifically intended for use with computer monitors at a fixed distance. People may have several pairs of glasses, one for each task or distance, with specific glasses for reading, computer use, television watching, and writing.