Spectacle lens types Explained
Choosing the right type of glasses
When choosing glasses, there are many different factors to consider:
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Age. For a person younger than 40, one pair of glasses with single vision lenses are usually all that is required. However, once a person reaches their early-to-mid 40's, different powers are required for distance vision and near vision. This can be given with either 2 pairs of single vision glasses, one for distance vision and one for near vision, with bifocal glasses, or with varifocal glasses.
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Lifestyle. For example, a regular desktop-computer user may prefer varifocals or separate pairs, as bifocals are not suitable for desktop computer use. Or a person who regularly travels to sunny climates may prefer photochromic (darkening in the sun) lenses.
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Level of correction required & size of selected frame. The stronger the glasses prescription and the larger the chosen frame, the thicker and heavier the corrective lenses will be. The weight and thickness of the lenses can be reduced by using high-tech thinner and lighter lens materials.
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Any pre-existing conditions. For example, a person with balance problems and dizziness may struggle with bifocals or varifocals.
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Other factors.
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Single Vision Glasses
Single vision glasses have lenses of a single power, with no separate areas of different powers, or areas that do not give clear vision at the intended distance(s):
Before the age of 40, one pair of single vision glasses to correct the distance vision is usually all that is required, as the eye can then focus to give clear vision at other distances.
After the age of 40, however, separate powers are required for distance and near vision. If single vision glasses are still desired, then 2 separate pairs are needed, one for each distance. The distance glasses are worn as they were previously, but the patient must switch to the near glasses whenever clear near vision is required.
Separate distance and near glasses are easier to adapt to than bifocals or varifocals. Distance glasses are also easier to walk in, since the wearer can see where they are putting their feet without reading areas getting in the way, and near glasses are the most comfortable option for long periods of near work, since the wearer has clear near vision in the whole lenses, rather than having to use the smaller bifocal or varifocal reading areas the whole time.
The main drawback of separate distance and near glasses is the inconvenience of carrying the second pair with you, and having to switch between them as needed.
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Bifocal Glasses
Bifocal glasses are glasses which contain the patient's distance power in the main part of the lens, but have visible segments in the lower part of the lens that contains the reading power, with a line separating the two. The idea is that the wearer looks through the top of the lens for clear distance vision, but looks down through the reading segments for clear near vision. There are 2 main types of bifocal lenses, known as D-segments and R-segments:
In D-segment bifocals, the reading areas are flat across the top, and semi-circular beneath. They get their name from the fact that they look like a letter "D" pointed downwards:
The main advantage of D-segment bifocals is that the wearer doesn't have to look down as far to get the full width of the reading segments. The main disadvantage is that the straight line across the top is more noticeable to other people.
In R-Segment bifocals, the reading areas are completely circular, and usually approximately 2.5 centimetres across. The R in their name stands for "Round":
The main advantage of R-segment bifocals is that the segments are less noticeable to other people than D-Segments. The main disadvantage is that the wearer has to look down further to get the full width of the reading segments.
The general advantages of bifocals are that they are more convenient than separate distance and near glasses, since they incorporate both the distance and near powers, they are easier to adapt to than varifocals, and usually cheaper.
The general disadvantages of bifocals are that they do not look as good as other lens types cosmetically, since the reading segments are visible to other people, there is no clear middle vision as the lenses jump straight from one power to the other, which means they are unsuitable for use with desktop computers, and they are difficult to walk in, since the reading segments prevent the wearer from seeing where they are putting their feet.
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Varifocal Glasses
Varifocal glasses are glasses which start with the patient's distance power at the top, and gradually change to the reading power as you go down the lenses. The part of the lens that gradually changes power is known as the varifocal corridor, and there are areas that do not give clear vision at any distance, known as areas of no useful vision:
(Note that in the following diagrams, the areas of no clear vision are indicated with black diagonal lines. This is simply for illustrative purposes, and these parts of the actual lenses do not look any different from the rest of the lenses).
There are many different makes and models of varifocal lenses. As a general rule, a lower quality, cheaper varifocal will have a narrower reading area and larger areas of no clear vision:
Whereas a higher quality, more expensive varifocal will have a wider reading area and smaller areas of no clear vision:
Varifocals are more versatile than either bifocals or separate distance and near glasses as, because they change power gradually, the wearer can see clearly at any distance if they look through the correct part of the lenses, though this does require practice.
Other advantages of varifocals are that they look better cosmetically than bifocals, since there are no visible segments, they are easier to walk in than bifocals (though not as easy as distance glasses), and they are more convenient than separate distance and near glasses.
The main disadvantages of varifocals are that they are harder to adapt to, there are more distortions in the peripheral (outside) vision, and they are usually more expensive than the other options.
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Comparison of glasses options
Here is a brief summary of the advantages and disadvantages of separate distance and near glasses vs. bifocal glasses vs. varifocal glasses:
1. Separate distance and near glasses:
Advantages:
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Easiest option to adapt to.
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Usually cheaper than varifocals.
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Distance glasses are easier to walk in than bifocals or varifocals.
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Near glasses are more comfortable for long periods of near work, since the wearer has clear near vision in the whole lenses, rather than just in the bifocal or varifocal reading areas.
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If a tint is required, to protect from bright lights, this can be put only in the distance glasses, and the near glasses can still be clear.
Disadvantages:
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Inconvenient, as the wearer must carry the near glasses with them and switch to them whenever good near vision is required, and then switch back again afterward.
2. Bifocal glasses:
Advantages:
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Convenient, as they contain both the distance and near powers, with no need to carry a separate near pair or switch between them.
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Easier to adapt to than varifocals.
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Usually cheaper than varifocals, and can be cheaper than separate distance and near pairs (usually more expensive than each pair separately, but cheaper than both pairs together).
Disadvantages:
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Harder to walk in, as the reading areas block the wearer's view of their feet when walking.
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No middle vision, just fixed distance vision and near vision, with a line separating the two.
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Not suitable for use with desktop computers.
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Do not look as good cosmetically, as the reading segments are visible to other people.
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In order to read, the wearer must keep their head still and look down with only their eyes. This is not ideal for people who prefer to look straight ahead at what they are reading.
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Not good for people with dizziness, or balance problems such as vertigo.
3. Varifocal glasses:
Advantages:
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The most versatile glasses option. Since they change power gradually, the wearer can see clearly at any distance by looking through the correct part of the lenses, although this does require practice.
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Better cosmetic appearance than bifocals. There are no visible reading areas, and the lenses look just like single vision lenses to other people.
Disadvantages:
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Harder to walk in, for the same reason as bifocals, although this is not as bad as it is with bifocals.
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Hardest glasses option to adapt to, and more distortions in the peripheral (outside) vision.
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Usually more expensive.
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As with bifocals, the wearer must keep their head still and look down with only their eyes in order to read.
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Again, not good for people with dizziness, or balance problems such as vertigo.
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Lens Upgrades
There are several optional upgrades available for spectacle lenses. These are usually available for separate distance and near glasses, bifocals, and varifocals, although they may not always be necessary or even desirable.
-High Index (thinner & lighter) lenses
High Index lenses are made of a material that is thinner and lighter for the same power than a lens made of standard lens plastic. These are ideal if your glasses need to be high-powered but you would prefer them not to be too thick or heavy. Different index levels are available, depending on the power and the level of thinning required.
High Index lenses are most often used to thin powerful short-sight correcting (diverging) lenses:
The high index option can also be used to thin long-sight and presbyopia correcting (converging) lenses, but these lenses can also be thinned by using a standard plastic lens of smaller diameter, so high index often isn't necessary:
-Photochromic lenses
Photochromic (often known as React-to-light) lenses are lenses which darken into sunglasses when exposed to direct sunlight, but have only a mild tint on them the rest of the time. They are usually available in either brown or grey:
When both clear glasses and prescription sunglasses are required, photochromic lenses can be considered a convenient 2-in-1 option. However, these lenses are not perfect and there are some drawbacks to consider:
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They do not darken very well in cars. This is because most of the ultraviolet light that they react to is blocked out by the car's windscreen and roof.
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They take much longer to lighten than they do to darken. Usually only around 30 seconds to darken, but more than 5 minutes to lighten again.
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There is a limit to how dark they can go, and they do not usually go as dark as prescription sunglasses.
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They also do not fully lighten. A small level of tint is visible on them even when are fully lightened.
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They tend to go dark even on mildly cloudy days. This is because most of the UV can still get through a thin layer of clouds.
-Hard Coating
A hard coating is a lens coating that does not alter the physical appearance of the lenses, but makes them more resistant to scratching. This usually comes as standard with varifocal or photochromic lenses.
-Anti-Reflection Coating
An anti-reflection coating is designed to prevent light bouncing off the lenses, and make more light pass through them. This is sometimes advised with particularly high index lenses, as these have more reflections than standard lens plastic and lower index lenses. However, the anti-reflection coating tends to mark, smudge and smear very easily, and requires a lot of effort to keep clean, therefore it is not suitable for everyone.