Corrective lenses: characteristics and benefits As the name implies, corrective lenses, also known as “prescription” lenses, are used in eyewear in order to correct defects in the vision of the wearer. As the name implies, corrective lenses, also known as “prescription” lenses, are used in eyewear in order to correct defects in the vision of the wearer. Corrective lenses: characteristics and benefits As the name implies, corrective lenses, also known as “prescription” lenses, are used in eyewear in order to correct defects in the vision of the wearer. Origins of the lens When we want to know the origins of the objects around us, a good place to start is often with the origins of the words we use to describe them. The word “lens”, according to the Oxford English Dictionary, is Latin for a genus of plants that includes the edible lentil (Lens culinaris), that tiny legume that is so popular in a number of Mediterranean countries. Indeed, the circular glass used in the first “spectacles”, produced in the 13th century by Venetian master glassworkers, was evidently similar in shape to that of the lentil, ** thicker in the middle and thinning out to the edges.** Although they may appear flat at first glance, lenses to this day all have either a convex or a concave surface in order to achieve the optical effect needed to correct the vision of the wearer. A lens is nothing more than a transparent material (glass or plastic) that uses this curvature of its surface to bend (or “refract”) light as it passes through the lens. Corrective lenses As the name implies, corrective lenses, also known as “prescription” lenses, are used in eyewear in order to correct defects in the vision of the wearer. Until about the 19th century, lenses were all either round or oval in shape. Today, though, they come in all shapes, sizes, and even colors, including for therapeutic effect. Photochromic lenses can even change color on their own in sunlight. Lenses can also be coated for scratch resistance or to ensure that your selfies are totally glare-free. Benefits of corrective lenses Corrective lenses are typically worn in front of the eye in order to improve vision. They are generally worn to correct refraction errors, the most common types of which are: myopia (or “near-sightedness”); hypermetropia (or “far-sightedness”); astigmatism; and presbyopia (related to aging of the eye). Types of lenses Positive lenses Positive (or “converging”) lenses have at least one convex surface and serve to “converge” light onto the focal point. They are often thicker at the center and get gradually thinner towards the edges. In this way, they are able to enlarge images and help to improve focus for people who suffer from far-sightedness, hyperopic astigmatism, or presbyopia. Negative lenses Conversely, negative (or “diverging”) lenses have at least one concave surface (i.e. thicker at the edges and thinner at the center) and are used to increase the focal length of the light as it passes through the lens. They help people suffering from near-sightedness or myopic astigmatism to see more clearly when objects are far away. Bifocal lenses Until just a few decades ago, people suffering from both near-sightedness and presbyopia were forced to alternate between two types of eyewear: one for seeing things up close and another to see more clearly at a distance. Over time, “bifocal” lenses were developed, which provide two different focal points to correct both defects. Bifocal lenses may be recognized by the small (square or rectangular) “window” in the lens, an effect caused by visible line that marks the joint between the two different lenses. The wearer then looks through one or the other of the lenses as needed. Although beneficial, bifocal lenses have a fixed field of vision and fail to take account of “intermediate” zones, which remain out of focus, and this can be a problem in situations such as driving. They can also cause eyestrain from having to keep switching between the two lenses. Over time, bifocal lenses have fallen out of use, in part because of how they tended to underscore the advanced age of the person wearing them. Progressive lenses Progressive lenses are multifocal lenses and, as such, are used to correct multiple defects. This advanced technology enables the lens to correct three distinct vision zones in a gradual progression, meaning that there is a gradual transition from one zone to the next. In this way, the wearer can see clearly at any distance. The field of vision is variable and is distributed progressively from the bottom of the lens to the top. Progressive lenses are typically prescribed to people over 40 who have begun suffering from presbyopia. There is a wide range of progressive lenses, and the most sophisticated feature a variable field of vision. The lower portion of the lens is used for seeing up close, while the upper portion is used for seeing more clearly at a distance. Unlike bifocals, progressive lenses have no lines separating the various zones, given that the transition from near, intermediate and distance vision is gradual. Each point on the surface of the lens has its own specific power, so this multifocal design is decidedly more advanced than the outmoded bifocals. Progressive lenses are primarily categorized by the quality of their progression corridor, with the highest quality lenses having the widest corridors.