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A Guide to Optical Lenses

A lens is an optical system with two refracting surfaces, the simplest lens has two spherical surfaces. Lenses are used to correct imperfections of the eye by increasing or reducing the focal length as needed. Optical lenses are made of transparent materials like plastic or glass. Each lens has two faces and is part of a sphere which can be convex or concave.

Convex Lenses

If a lens is thicker at the center than at the edges, it is a convex lens or converging lens. Parallel rays passing through these lenses will converge to meet at a focus point. These lens causes magnification of an object while viewing through them and are also called + (plus) lenses.

Concave Lenses

If a lens is thinner in the center and thicker at the edges, it is a concave or diverging lens. Parallel rays of light passing through these lenses will spread out. Concave lenses cause an object to look very miniscule when viewed through it. These lenses are also called - (minus) lenses.

Lens Categories

The different kinds of lenses can be used as eyeglasses. The specific category recommended depends on several factors such as lifestyle, kind of work, preference and visual needs etc. Based on these needs they are broadly classified as:

Single Vision Lenses

This lens type has one prescription that covers the whole lens. They're typically for people who require one kind of correction, i.e long or short sight. Such users need glasses for activities like driving where they need to see distances or reading where they need to see close up objects.

Bifocal Lenses

Bifocal lenses are divided into two sections to accommodate a combination of any two different prescriptions into one lens. The upper area of the lens will have one prescription (usually distance) and a segment in the lower half will have the other (usually near vision). You can clearly see a demarcation line between the two sections in these lenses.

Progressive Lenses

Progressive or Varifocal lenses allow you to see at different distances with the same lens. They have different sections for viewing close up, distances and intermediate. So whether you are driving, using a computer, shopping or reading, you can wear one pair of varifocals to accommodate all your vision needs. The different sections of the lens are not clearly noticeable for people looking at the wearer.


Lens Materials

Mineral Glass

Made of silica these lenses are highly scratch resistant with excellent optical qualities. However they are low in impact resistance and can lead to injuries to eyes while worn as spectacle lenses, They are not in use for optical lenses, however because of good optical qualities they are still being used in medical instruments.

MR 8 Lenses

These lenses are also called MITSUI RESIN lenses and have a refractive index of 1.60. They are lighter, stronger and clearer optical lenses. MR 8 lenses are made of thiourethane resin which makes it possible to build thin eyeglasses with high impact resistance. Since they resist breaking and chipping, they are suggested for rimless frames as they are safer to use.

Plastic or CR 39

These lenses are made of cellulose acetate and are superior to mineral glass in terms of softness and less weight. They can be tinted or colored for cosmetic appeal. CR stands for Columbia Resin and its 39th formula of a thermosetting plastic. The material is highly impact resistant, carries high transparency and multiple coatings are possible. The only drawback with these lenses is that they are easily prone to getting scratched.

Polycarbonate Lenses

These lenses are made from derivatives of petroleum and plastic polymer and are 10 times more impact resistant than CR 39 lenses. The refractive index of polycarbonate lenses can be 1.59 and this provides an extra level of protection for users. These lenses are often used for children, sportswear and in rimless frames. All polycarbonate lenses are 100 % UV protected.

Trivex Lenses

These lenses are impact resistant with a refractive index of 1.532. They are similar to polycarbonate lenses but have higher impact resistance and better optical quality which means clearer vision. Although the material is lighter, it is thicker than polycarbonate hence cannot be used for high prescriptions.

Lens Thickness and Refractive Index

Refractive index is a measure of how efficiently a lens material can bend light. This index of refraction of eyeglass lens material usually ranges from 1.50 (regular plastic) to 1.74 of high index or compressed plastic. The higher the index the thinner the lens material.

A polycarbonate lens with an index of 1.59 would be 40% thinner than a regular 1.50 index plastic lens and the extremely high index lens of 1.74 would be 65% thinner than a 1.50 index plastic lens.

Want to learn more about Lens Extras and Treatments?

Our extensive range of lenses always include scratch-resistant treatment as standard. However we’ve also got a great range of lens extras and treatments that will help to protect or enhance your lenses. Have a look at what we offer below.

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