Lenses come in many flavours and again, at first, things can become a little confusing. But don’t worry. I am here to explain them all in detail so that you can understand and choose the best lens option for you.

Firstly lenses can be made in two materials Organic (Plastic) or Mineral (Glass).

Most lenses sold today are of the plastic variety as they are far lighter than glass. In some rare occasions when extremely thin lenses are required for very high prescriptions glass can be the best option.

Each lens will also have what we call a “Refractive index” The way in which the refractive index is calculated is not important at this time. What is important is what it means to the thickness of the lens.

Most people think that when we say a 1.6 lens that this means it is going to be 1.6mm thick, THIS IS WRONG. Basically the higher the number the thinner the lens will be. This is because materials that bend light more than others have a higher “Refractive index” and hence a shorter or longer focal length. This means that the same correction and focal length can be achieved from “less” lens material making them thinner.

Here is a key to choosing the best lens.

Standard Plastic CR39 (1.5)
Ideal for most prescriptions (prescriptions with powers stronger than +3.00 and -3.00 will start to look thick, powers between the two will look fine). Plastic (1.6)
An ideal option for prescriptions up to +5.00 and down to -5.00. 1.6 lenses are 23% thinner than standard plastic. Prescription outside this range will start to look thick.Plastic (1.67)
Probably the most popular of the thinner lens options. 15% thinner than the 1.6 and 25% flatter. Ideal for prescriptions up to +7.00 and down to -7.00 prescription outside this range will look thick. Plastic (1.74)
The thinnest plastic lens available on the market. 50% thinner than the 1.5 standard plastic and 30% lighter than the glass equivalent. Great for mid to high prescriptions. Glass (1.7)
An ideal budget thin lens for “-“ power prescriptions up to – 10.00. However, they will be heavier than plastic. Glass (1.8)
Rarely used but good for high power prescriptions. Expensive option. Glass (1.9)
Rarely used but excellent for high power prescriptions. Expensive option.

Lens Types

There are also many different styles of lenses. Lenses for single vision correction e.g. distance or reading, to Bifocals and Varifocals for the correction of both distance and reading within one pair of glasses.

Varifocals

Varifocals blend gradually from your distance prescription through your intermediate prescription in to your reading prescription. This means that the lens has to be set at exactly the correct height in the frame to start with.

The crucial measurement required to align varifocals is the height in millimeters from the centre of your pupil to the bottom of the new frame, this measurement cannot be taken online.

Although it is becoming common practice online suppliers to use averages and experience along with other factors such as order information to calculate some measurements such as the “PD” it is not yet possible to do this with Varifocals.

Some online companies will still provide varifocals for you and you may get lucky. HOWEVER I would not advise anyone to do this until technology allows us to take the accurate measurements needed.

We are currently working on a varifocal system that should allow us to supply Varifocals accurately. I will let you know when the results are in.

Bifocals

Bifocals are basically distance glasses with a reading segment in the bottom. Unlike Varifocals Bifocals are commonly sold online as the height measurement is far less crucial and averages can be used with great success. Due to the fact that there is no gradual phase from distance into reading and they are just distance with a reading segment Bifocals have a far wider corridor for reading than Varifocals but have no intermediate correction.

There are also various types or “shapes” of the reading segment.

Typically the standard is the “D Seg” this one looks like the letter D on its side with the flat part it the top hence the name D Seg.

There is also a “round” segment in various sizes and executive or “E Type” bifocal where the whole bottom half of the lens is dedicated to reading (Great for people who do a lot of reading at work using plans etc).

Bifocals are only available in standard plastic, standard glass and 1.6 plastic.

Single Vision

“Single Vision” lenses can be used for just distance, just intermediate or just reading hence the name “single vision”. However, many people who are short sighted “-” Power SPH prescriptions will be able to read with their distance glasses. Single vision lenses are available in both materials and all indexes.

Article written by: Mark Bowden.
For more information about prescription glasses visit: spex4less.com/