Corrective lens

A corrective lens is a lens typically worn in front of the eye to improve vision. The most common use is to treat refractive errors: myopia, hypermetropia, astigmatism, and presbyopia. Glasses or 'spectacles' are worn on the face a short distance in front of the eye. Contact lenses are worn directly on the surface of the eye. Intraocular lenses are surgically implanted most commonly after cataract removal, but can be used for purely refractive purposes. A corrective lens is a lens typically worn in front of the eye to improve vision. The most common use is to treat refractive errors: myopia, hypermetropia, astigmatism, and presbyopia. Glasses or 'spectacles' are worn on the face a short distance in front of the eye. Contact lenses are worn directly on the surface of the eye. Intraocular lenses are surgically implanted most commonly after cataract removal, but can be used for purely refractive purposes. Corrective lenses are typically prescribed by an ophthalmologist or an optometrist. The prescription consists of all the specifications necessary to make the lens. Prescriptions typically include the power specifications of each lens (for each eye). Strengths are generally prescribed in quarter-diopter steps (0.25 D) because most people cannot generally distinguish between smaller increments (e.g., eighth-diopter steps / 0.125 D). The use of improper corrective lenses may not be helpful and can even exacerbate binocular vision disorders. Eyecare professionals (optometrists and ophthalmologists) are trained to determine the specific corrective lenses that will provide the clearest, most comfortable and most efficient vision, avoiding double vision and maximizing binocularity. Ready-made single-vision reading glasses go by many names, including over-the-counter glasses, ready readers, cheaters, magnifiers, non-prescription readers, or generic readers. They are designed to lessen the focusing burden of near work, such as reading. They are typically sold in retail locations such as pharmacies and grocery stores, but are also available in book stores and clothing retailers. They are available in common reading prescriptions with strengths ranging from +0.75 to +3.50 diopters. While these 'magnifiers' do indeed make the image of the viewed object bigger, their main advantage comes from focusing the image, not magnification. These glasses are not tailored to a person's individual needs. A difference in refractive error between the eyes or presence of astigmatism will not be accounted for. People with little to no need for correction in the distance may find off-the-shelf glasses work quite well for seeing better during near vision tasks. But if the person has a significant need for distance correction, it is less likely that the over the counter glasses will be perfectly effective. Although such glasses are generally considered safe, an individual prescription, as determined by an ophthalmologist or optometrist and made by a qualified optician, usually results in better visual correction and fewer headaches and visual discomfort. Another criticism of over the counter glasses is that they may alleviate symptoms, causing a person to forgo the other benefits of routine vision exams, such as early diagnosis of chronic disease. Although lenses are normally prescribed by optometrists or ophthalmologists, there is evidence from developing countries that allowing people to select lenses for themselves produces good results in the majority of cases and is less than a tenth of the cost of prescription lenses. Single vision lenses correct for only one distance. If they correct for far distance, the person must accommodate to see clearly up close. If the person cannot accommodate, they may need a separate correction for near distances, or else use a multifocal lens (see below). Reading glasses are single vision lenses designed for near work, and include over the counter glasses. They come in two main styles: full frames, in which the entire lens is made in the reading prescription, and half-eyes, style glasses that sit lower down on the nose. Full frame readers must be removed to see distance clearly, while the distance can be clearly viewed over the top of half-eye readers. A bifocal is a lens with two sections, separated by a line (see image to the right). Generally, the upper part of the lens is used for distance vision, while the lower segment is used for near vision. The area of the lens that caters to near vision is called the add segment. There are many different shapes, sizes, and positions for the add segment that are selected for functional differences as well as the visual demands of the patient. Bifocals allow people with presbyopia to see clearly at distance and near without having to remove the glasses, which would be required with single vision correction. Trifocal lenses are similar to bifocals, except that the two focal areas are separated by a third area (with intermediate focus correction) in the middle. This segment corrects the wearer's vision for intermediate distances roughly at arms' length, e.g. computer distance. This lens type has two segment lines, dividing the three different correcting segments.