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.